Characteristics of As-Cast and Subcriticaiiy Heat- Treated High-Chromium- Molybdenum White Irons for Thick-Section Castings

J. L. ParksSenior Research Assistant Climax Molybdenum of Michigan Subsidiary of A MAX Inc Ann Arbor, Michigan

ABSTRACTThe hardness, microstructure, abrasive wear resistance and mechanical properties were evaluated for Sour high-chromium- molybdenum irons in the as-cast condition and after various subcritical heat treatments. After pouring, the irons were cooled to simulate the mold cooling rates in either 100- or 150-mm (4- or 6-in.) -thick plates. Compositions were chosen to provide matrix microstructures which did not contain ferrite-carbide transformation products in the mold-cooled condition. When cast and cooled as thick sections, matrix microstructures of the as-cast high-chromium-molybdenum irons contained austenite and significant quantities of martensite. Hardness of the as-cast austenitic-martensitic irons was higher than the hardness obtained for predominantly austenitic irons cast in thinner section sizes. Hardness of the irons increased after subcritical heat treatment at around 500C (930F). As-cast and subcriticaiiy heat-treated thick-section irons exhibited excellent abrasion resistance similar to that exhibited by martensitic (fully heat treated) high-chromium-molybdenum irons. Mechanical properties of the austenitic-martensitic irons were also comparable to those typically exhibited by martensitic high- chromium-molybdenum irons.

IntroductionLarge tonnages of high-chromium-molybdenum (Cr-Mo) irons are currently being produced for castings which require a combination of abrasive wear resistance and toughness not obtainable in other alloyed white irons or steels. These castings are being used in equipment for the mining industry, coal and mineral processing, the cement industry and in other large castings such as rolling mill rolls. High-Cr-Mo irons are giving reliable performance in large impact hammers, mill liners and pulverizer rolls, applications where other alloyed white irons have proven too brittle. They have also replaced considerable tonnages of steel castings which had good toughness but lacked adequate abrasion resistance.

Experience has shown that high-Cr-Mo irons exhibit the best combination of abrasion resistance

and toughness, particularly resistance to spalling and fracture under conditions of severe repeated impact, when they are heat-treated at high temperatures (950-1060C (1750-1940F)) and cooled (or quenched) to obtain a martensitic matrix microstructure. However, as-cast irons with either austenitic or austenitic- martensitic matrix microstructures have been used successfully for castings which do not encounter severe impact in service. The most obvious advantage of using high-Cr-Mo irons in the as-cast condition is the cost savings and energy conservation that results from eliminating the high-temperature heat treatment. In addition, less complex production techniques can be used for producing large, intricate castings which ordinarily show a tendency to crack during conventional high-temperature heat treatments.

High-Cr-Mo irons are used for abrasion-resistant castings ranging in size from very small — 12-mm (0.5-in.) -diameter — grinding balls up to massive — 355-mm (14-in.) -thick — table segments for roller pulverizers but in recent years more attention has focused on production of thick-section castings. Accordingly, the characteristics of four as-cast high-Cr-Mo irons produced as thick sections were studied in this investigation. After pouring laboratory heats of the irons, castings were cooled to simulate the cooling rates in either 100- or 150- mm (4- or 6-in.) -thick plate castings. Microstructure, hardness, abrasive wear resistance and mechanical properties were determined for the irons in the as-cast condition and after tempering at 205C (400F). Hardness, amount of retained austenite, abrasive wear resistance and mechanical properties were also determined for the irons after heat treating at subcritical temperatures in the range 450-550C (840-1020F).Selection of AlloysFor as-cast high-Cr irons, hardenability can be defined as that property which determines the amount of various ferrite- carbide transformation products formed when austenite transforms upon cooling in the mold from high temperature. Since high-Cr irons with ferrite-carbide (pearlitic) matrix microstructures have much lower abrasion resistance than irons with austenitic or martensitic matrix microstructures, high-Cr irons to be used in the as-cast condition must have enough hardenability to suppress transformation of austenite to ferrite- ' carbide structures during mold cooling. Carbon (C) and Cr influence hardenability to a limited extent in as-cast high-Cr irons. Studies' have shown that increasing C content generally decreases hardenability while increasing Cr content increases hardenability. For example, in 50-mm (2-in.) Y-block castings, an iron containing nominally 2.7% C - 27% Cr will have predominantly austenitic matrix microstructure in the as-cast condition while irons containing 2.7% C and either 15 or 20% Cr will have pearlitic matrix microstructures.

In larger section sizes increasing Cr content alone does not provide enough hardenability and additions of other alloying elements are therefore required to control hardenability. Earlier

investigations1'2 of as-cast high-Cr irons have shown that adding Mo, copper (Cu) and nickel (Ni) or increasing manganese (Mn) content increases hardenability but that increasing nitrogen (N) (above approximately 0.07%) or silicon (Si) content decreases hardenability.

Based on the available information on hardenability, the following compositions were chosen for the experimental irons to provide enough hardenability for obtaining as-cast microstructures without ferrite-carbide transformation products in

either 100- or 150-mm (4- or 6-in.) -thick plate

castings.

Since previous studies3 of as-cast austenitic high-Cr-Mo irons indicated that increasing Si content improves fracture toughness, irons containing nominally 0.5 and 1.2% Si were produced for each section size. In the !00-mm (4.-in.) -thick plates, moreMn, 1.5%, was added to the iron containing 1.2% Si to offset the loss in hardenability resulting from the higher Si content. Composition of irons for the 150-mm (6-in.) -thick plates was adjusted to provide enough hardenability by adding 1.5% Mn, decreasing C content to 2.6% and increasing Cr and Mo contents to 20 and 2.5%, respectively.Experimental ProceduresMelting and Casting

The high-Cr-Mo irons studied in this investigation were produced by induction melting 61-kg (135-lb) heats in an alumina crucible. The charge materials consisted of electrolytic iron, graphite, chromium, ferromolybdenum, ferromanganese, ferrosulfur, ferrophosphorus, copper and nickel. After meltdown the irons were superheated at 1565C (2850F) for 5 min, cooled to 15IOC (2750F) and poured into a chill-based chemical analysis sample mold and into oil-bonded sand Y- block molds with leg dimensions of 50 x 65 x 190 mm (2 x 2.5 x 7.5 in.). The Y-block castings were removed from the mold about 10 min after pouring, transferred to a furnace at I095C (2000F) and program cooled to simulate the mold-cooling rates in either 100- or 150-mm (4- or 6-in.) -thick plate castings. Cooling rate curves are shown in Fig. 1.

Chemical AnalysisThe C content of each heat was determined using material from a pin sample aspirated from one of the Y-blocks.

Balance of the composition was determined using pulverized material from the cast chemical analysis sample. Chemical compositions of the irons are reported in Table 1.Subcritical Heat Treatments Samples of the experimental irons were given various subcritical heat treatments in the temperature range 205-550C (400-1020F). These treatments were carried out in an electric furnace with no protective atmosphere. In one series of heat treatments, samples were placed in a furnace at the desired temperature, held at this temperature for various lengths of time, removed from the furnace and cooled in still air to room temperature. Samples were also heat treated by placing them in a furnace at 205C (400F), heating to the desired temperature at 12°C/hr (22°F/hr), holding for various lengths of time, furnace cooling at 12°C/iir (22°F/hr) to 205C (400F) and cooling to room temperature in still air. Slower heating and cooling rates were chosen to simulate those encountered in the heat treatment of very large castings.Hardness and MetallographyViekers hardness (30-kg load) was measured on sample surfaces which had been ground through 600-grit silicon carbide paper and polished. Metallographic examinations were performed by light microscopy and in some cases by preparing carbon replicas of the polished and etched

Fig. 1. Cooling rates used to simulate mold cooling of 100-

and 150-mm (4- p»andve-iftOw-thick, Platttci^Ar_-iii «»|Wlt|»ri «

Element, % 100-mm (4-in.) Thick Plate

150-mm (6-in.) Thick

Plate

Carbon 3.0 2.6Silicon 0.5 and 1.2 0.5 and 1.2Manganese 0.8 and 1.5 1.5Chromium 18.0 20.0Molybdenum

2.0 2.5

Copper 1.0 1.0

surfaces and examining them using electron microscopy.

Table 1. Chemical Composition of the Experimental Irons

Heat

Section Thickness, mm (in.)

Element, X

No. C Si Mn Cr Mo Cu Ni

5670

100 (4) 3.03

0.48

0.83

17.8

1.96

1.00

0.15

5757

2.95

1.17

1.62

17.9

1.92

1.01

0.14

5813

150 (6) 2.65

0.54

1.45

19.7

2.49

1.01

0.16

5816

2.66

1.13

1.54

19.7

2.40

0.98

0.13

Measurement of Retained AusteniteThe amount of retained austenite in selected irons has been measured using an x-ray diffraction method devised for highly textured alloy cast irons.4 With this method the specimen is given a conventional metallographic polish, lightly etched, then analyzed in a diffractometer using filtered Mo-Ka radiation. A unique feature is the diffractometer specimen holder which simultaneously rotates and tilts the specimen, to essentially eliminate anomalously high or low diffraction peak intensities due to preferred orientation in the structure. X-ray diffraction data is used to calculate the volume percent retained austenite in the metallic matrix. Eutectic carbides, secondary carbides and carbides formed during austenite decomposition are excluded from the volume from which the amount of retained austenite is calculated.

Abrasion ResistanceResistance to high-stress abrasive wear was determined by a pin- abrasion test apparatus described by Muscara.5 The test uses a specimen in sliding contact with bonded abrasives to simulate the type of abrasion in which the forces involved are large enough to crush or fragment abrasive particles. In the pin test, the end of a 6.35-mm (0.250-in.) -diameter specimen, under an axial load of 6.2 kg (15 lb), is simultaneously rotated about its axis (20 rpm) and moved back and forth in a nonoverlapping pattern across fresh abrasive cloth. The specimen travels a total distance of 12.8 m(504 in.) in approximately 7 min. The first test is a run-in to equilibrate the surface characteristics of the specimen to the abrasive. Weight loss is determined for each of two additional tests on fresh abrasive cloths and the results averaged.

Mechanical PropertiesFour laboratory tests were performed on selected experimental irons to characterize their toughness. sThese tests measured compressive strength, static and dynamic fracture toughness and impact-bend tensile strength.Compressive Strength — Cylindrical specimens measuring 10 mm (0.4 in.) in diameter by 20 mm (0.8 in.) in length were prepared and compressed in a testing machine at a rate of 12%/hr until fracture. Strain was measured with foil-strain gages attached to the specimen. Three tests were usually performed for each iron and reported results are the average value.

Fracture Toughness — Static fracture toughness, Kic, was determined using compact tension specimens with the dimensions shown in Fig. 2. After generating the fatigue crack, Kic values were determined using the test procedure specified in ASTME-399-74, "Plain-Strain Fracture Toughness of Metallic Materials." The reported Kic value for each iron is the average value of three tests. For determining dynamic fracture toughness, Kid, 10 x 10 x 55-mm (0.394 x 0.394 x 2.165-in.) specimens were prepared and notched by electrical discharge machining a 0.2-mni (0.01-in.) -radius slot 2 mm (0.08 in.) in depth, as shown in Fig. 3. Specimens were tested in an instrumented charpy impact machine which recorded load as a function of time

during impact. The hammer was released from a

drop height which produced a hammer velocity of 3.5 m/sec (11.5 ft/sec) at impact. The Kid values were calculated from the fracture load (maximum load during impact) using the equation given in ASTM E-399-74 for bend specimen geometry.Impact-Bend Tensile Strength — Specimens measuring 10x10 x 55 mm (0.394 x 0.394 x 2.165 in.) were used to determine impact-bend tensile strength. Specimens were broken in an instrumented charpy impact machine and the fracture load (maximum load during impact) was recorded. Fracture load was used to calculate impact-bend tensile strength irom the equation:

a = (6M/W:B)where M is the bending moment, W is the width and B is the thickness of the specimen. Two tests were performed for each iron and the results were averaged.

Results and Discussion Microstructure and HardnessBefore discussing the experimental results, it may be helpful to briefly review the physical metallurgy of high-Cr irons. A general knowledge of the phase relations in the Fe-Cr-C ternary system has evolved from numerous studies conducted over the past half century. Practical applications of the Fe-Cr-C equilibrium

diagram are limited to the extent that equilibrium

Fig. 3. Dimensions of the specimens used for determining dynamic fracture toughness, Kid.

Fig. 2. Dimensions of the compact tension specimens used for determining static fracture

toughness, Kic.

conditions are virtually never encountered in the production of castings and that the addition of Mo and other alloying elements alters the phase relations observed in Fe-Cr-C alloys. The extent to which phase relations are altered has not been fully documented, paiticularly for high-C, high-Cr alloys. Molybdenum and Mn are soluble in M7C3 carbides and significant amounts of Mo partition to the M7C3 phase. Molybdenum-rich M?C eutectic carbides and MsC carbides have been observed in high-Cr-Mo irons but are present in small quantities. Molybdenum and M11 are also soluble in M3C carbide. For the most part, the alloying elements Si, Ni and Cu have limited solubility in the carbide phases and therefore are concentrated in the metallic matrix.

High-Cr irons are characterized by the presence of hard, relatively discontinuous M7C3 carbides, in contrast to the softer M3C carbides or continuous M3C carbide eutectic in alloyed white irons containing lower Cr contents. Referring to the liquidus surface of the Fe-Cr-C metastable system6 shown in Fig. 4, the compositions used for high-Cr irons are chosen so that C and Cr contents are within the austenite (7) or M7C3 liquidus field boundaries and so that binary eutectic solidification of either y + M3C or y + a is avoided. Compositions which meet these two requirements are indicated by the shaded area in Fig. 4. The major classifications of high-Cr irons, such as those in ASTM A 532-75, "Abrasion-Resistant. Cast Irons," specify compositions that fall within the main subdivisions of the ranges of possible C and Cr contents. As indicated by Fig. 4, solidification in hypereutectic high-Cr irons — those with compositions within the M7C3 carbide liquidus field boundaries — occurs by crystallization of primary M7C3 carbides followed by freezing of the y 4- M7C3 carbide eutectic. In hypoeutectic high-Cr irons — those with compositions within the austenite liquidus field boundaries — solidification occurs by crystallization of austenite dendrites followed by freezing of the y + M7C3 carbide eutectic.

Austenite.is stable at high temperatures (see the isothermal sections7 shown in Fig. 5) but upon cooling under equilibrium

conditions it will transform to ferrite plus carbide at some temperature above 700C (1290F). However, when cooling under nonequilibrium conditions such as those normally encountered in foundry production, austenite becomes supersaturated in C and Cr. This factor, along with the presence of other alloying elements which affect the transformation kinetics of austenite, such as Mo, Mn, Ni and Cu, can result in austenite being

retained to room temperature. Chemical composition jnd cooling rate determine whether the austenite formed during s61idification will be retained to room temperature, decompose partially or fully to ferrite-carbide transformation products or, in some circumstances, partially transform to martensite.

Two basic requirememt&o^y>^.metioc transformationofc« austenite to martensite to occur in high-Cr irons. First, C content in the matrix must be lowered from the high levels present following solidification and second, there must be adequate hardenability to avoid the transformation of austenite to ferrite-carbide constituents such as pearlite or bainite. The first requirement usually involves precipitation of secondary carbides either during slow cooling in the mold or during a subsequent high-temperature heat treatment. Slow cooling rates in thick-section castings provide an opportunity for diffusion of C from the matrix at high temperatures and precipitation of secondary carbides; therefore, some martensite is usually present in thick-section castings in the as-cast condition. Small amounts of martensite are also found in the eutectic regions of as-cast, thin-section high-Cr irons, probably due to depletion of Cr and C in the austenite matrix adjacent to the M7C3 carbides. In most circumstances, martensitic microstructures in high-Cr irons are obtained by heat treatment. As described by Maratray,8 after heating to high temperature and precipitating secondary carbides in the matrix, austenite can transform to martensite upon cooling, providing the iron has enough hardenability to avoid forming ferrite-carbide transformation products.

Mierostructure of the Experimental Irons — The experimentalhigh-Cr-Mo irons exhibited matrix microstructures which were markedly different from those exhibited by irons with similar compositions but cast in thinner section sizes. High-Cr-Mo irons with enough hardenability to avoid transformation to ferrite-carbide constituents typically have predominantly austenitic microstructures, as illustrated in Fig. 6, when cast in section sizes on the order of 50 mm (2 in) or less. When cast in 50- mm (2-in.) Y-blocks and then program cooled to simulate either 100- or 150-mm (4- or 6-in.) -thick cast sections, the exper imental irons had matrix microstructures containing more martensite in the regions of eutectic solidification and precipitation of secondary carbides along with some martensite transformation in the center of the austenite dendrites, as illustrated in Fig. 7. All the experimental irons had enough hardenability to substantially avoid transformation to ferrite- carbide aggregates during simulated mold cooling. The dark etching regions in the center of the austenite dendrites (Fig. 7) resemble pearlite upon examination by light microscopy; however, careful examination by electron microscopy revealed that these areas comprised a dense precipitation of fine secondary carbides in a mixture of austenite and martensite.

The difference in microstructure exhibited by as-cast high-Cr- Mo irons cast in thin and thick section sizes (fast and slow cooling rates) — i.e. more martensite transformation and precipitation of secondary carbideis for irons cast in thicker, more slowly cooled sections —

Fig. 4. Liquidus surface of the Fe-Cr-C meiastabie system.6

Fig. 5. 900 and 1150C (1650 and 21 OOF) isothermal sections of the Fe-Cr-C system.7

was previously reported by Gundlach.2 In thick-section castings the slower cooling rate allows more time for diffusion of C and precipitation of carbides which lowers the matrix C content; thus, Ms temperature is raised and more martensite transformation can occur. The amount of retained austenite in several experimental irons is reported in Table 2. Matrix microstructure of the experimental

«v

Table 2. Retained Austenite and Hardness of the Experimental As-Cast Irons

Heat No.

SectionNominal Composition Thickness.

mjn (in.)

Retained3

Austenite, Z

Hardness, HV30

5670.

5757

3C-18Cr-2Mo (0.A8Si)100 (4)

3C-18Cr-2Mo (1.17Si)

48 50 659 640

5813

5816

2.6C-20Cr-2.5Mo (0.54Si)150 (6)

2.6C-20Cr-2.5Mo (1.13Si)

46 30 630 640

irons contained 30-50% retained austenite, while the balance of the matrix microstructure is martensite, as compared to the almost entirely austenitic matrix microstructures typically obtained in as-cast high-Cr-Mo irons cast in thinner section sizes.

Hardness of Experimental Irons — As shown in Table 2, hardness values of the experimental as-cast irons with austenitic-martensitic matrix microstructures were within the range 630-660 Vhn 30. In comparison, as-cast irons with similar compositions but predominantly austenitic matrix microstructures typically exhibit hardness values of 470-540 Vhn 30.2

#

50jjum

Volume percent of the metal-lie. matrix.

Fig. 7. Austenitic-martensitic matrix microstructure typical of the experimental as-cast high-chromium-molybdenum irons program cooled to simulate mold cooling of 100- lo 150-mm (4- to 6-in.) -thick plates: a) HCI picral etch, X500; b) carbon replica, X5000.

pig. 6. Predominantly austenitic matrix microstructure typical of as-cast high-chromium-molybdenum irons cast as 50-mm (2- in.) Y-Blocks: a) HCI picral etch, X500; b) carbon replica, X5000.

Response to Subcritical Heat Treatment Changes in hardness and microstructure were observed when the experimental austenitic-martensitic irons were isothermally heat treated at subcritical temperatures. Figure 8 and Table 3 illustrate hardness of the 2.6 C-20 Cr-2.5 Mo irons after rapidly heating to temperatures in the range 205-550C (400-1020F),

Subcritical Heat

TreatmentHardness, HV30

Temperature, C (F)

Time, hr

Heat 5813

(0.542 Si)

Heat 5816

(1.132 Si)

As-Cast*3

—

630 640

4 610 631

205 (400)

8 606 631

16 602 ' 6264 615 657

290 (555)

8 601 636

16 601 651-

4 620 685

450 (840)

8 618 693

16 633 6764 668 728

500 (930)

8 683 723

16 677 6914 629 646

550 (102 0)

8 625 602

16 620 582

''Program cooled to simulate a 150-mm (6-in.) thick plate.

holding for 4, 8 or 16 hr and air cooling. A significant increase in hardness was obtained by isothermally treating at about 500C (930F). Hardness values obtained by subcritical heat treatment depended on temperature, time and composition of the iron; ^ maximum hardness obtained in the 1.13% Si iron was highen than that obtained in the 0.5% Si iron.

A second series of subcritical heat treatments involved heating the irons at a much slower rate, 12°C/hr (22°F/hr) to temperatures in the lange 450-550C (840-1020 F), holding for 5 hr, cooling at 12° C/ hr (22° F/ hr) to 205C (400F)

and air cooling to room temperature. Changes in hardness resulting from these treatments, shown in Fig. 9 and Table 4, were similar to those observed in the first series of subcritical heat treatments. Changes in hardness were influenced by treatment temperature and rates of heating and cooling, as well as chemical com-position of the iron.

In addition to affecting hardness, subcritical heat treatments resulted in some transformation of retained austenite, as shown in Table 5. As subcritical heat treatment temperature was increased, more retained austenite was transformed.

Experimental results show that subcritical heat treatments influence both hardness and microstructure in as-cast high-Cr- Mo irons with austenitic-martensitic matrix microstructures. Subcritical heat treatments at around 500C (930F) generally produced an increase in hardness and a reduction in the

amount, of retained

austenite. As- cast irons with austenitic-martensitic\ microstructures respond much differently to subcritical heat treatments than as-cast irons with fully austenitic microstructures. For example, Maratray1

reported no change in the microstructure of as-cast austenitic irons that were held at 500C (930F) for up to 30 hr. Response of the as-cast austenitic- martensitic irons to subcritical heat treatments appears to be similar to the secondary hardening phenomenon exhibited by some quenched-and-tempered alloy steels and observed most recently by Maratray' for heat-treated high-Cr-Mo irons with austenitic-martensitic microstructures obtained by reaustenitizing and quenching.

Results indicate that hardness and amount of retained austenite after subcritical heat treatment is a function mainly of treatment temperature and time but also depends c i chemical composition of the iron. Additional study will be required to fully understand the relationship between these variables and their effect on hardness and retained austenite; however, several factors probably account for the observed changes in hardness. First, some of the observed increase in hardness may be due to

Table 3. Hardness of the 2.6 C-20 Cr-2.5 Mo Irons after Sub- critical Heat Treatment3

at temperature for the indicated time and air cooled.

Fig. 8. Hardness of the 2.6 C-20 Cr-2.5 Mo irons after rapidly heating to temperatures of 205-550C (400-1020F),

L4_8,an<M6.hrandair cooling. ..*.................

precipitation of Fine alloy carbides in the martensite during subcritical treatment. Second, transformation of retained austenite, either to ferrite-carbide constituents at the subcritical treatment temperature or to martensite upon cooling, may influence hardness. Influence of the transformation of retained austenite on hardness will depend on the type and amount of transformation products. Previous studies at the author's firm have shown that subcritical treatments at higher temperatures than those used in this investigation, in the range 570-625C (1060-1155F), generally resulted in transformation of retained austenite to a relatively coarse ferrite-carbide aggregate with relatively low hardness, as shown by the data in Fig. 10.

Abrasive Wear ResistanceResults of high-stress abrasive-wear tests conducted on 100-and 180-,um particle size (l 50- and 80-mesh) Jo met abrasive are summarized in Table 6. Irons were tested iri two conditions — after tempering at 205C (400F) for 4 hr and after treating for 5 hr at a temperature selected from the subcritical heat treatment experiments to provide maximum hardness. Abrasive wear (weight loss) of the irons tempered at 205C (400F) ranged from 18.4 to 26.1 mg when tested on 100-/jm garnet and from 58.4 to 69.2 mg when tested on 180-^m garnet. Irons exhibited better abrasion resistance after subcritical heat treatment. For example, abrasive wear of the subcriticaiiy heat-treated irons ranged from 48.8 to 52.3 mg when tested on 180-/um garnet.

Heat Treatment Subcritical Temperature, C (F)

Hardness, HV303C-18Cr-2Mo Irons

2.6C-20Cr-2

.5Mo Irons

Heat 567

0 (0.48% Si)

Heat 575

7 (1.1

7* Si)

Heat 5813

(0.54% Si)

Heat 581

6 (1.13% Si)

-

As-Cast 659b 640b 630c 640c

450 (840) 660 675 604 721

475 (885) 661 675 633 728500 (930) 675 695 670 670525 (975) 709 699 688 639550 (1020)

697 658 665 611

aHeated Co and cooled from the subcritical treatment temperature at 12 C/hr (22 F/hr)Program cooled to simulate a lOO-mm (4-in.) thick plate.

Program cooled to simulate a 150-mm (6-in.) thick plate.CVolume percent of the metallic matri

Table 5. Hardness

and Amount of

Retained Austenite for the 2.6

C-20 Cr-2.5 Mo Iron

s after Subcritical

Heat Treatment

for 5 Hr at

Temperatures of 450-500C

(840-

1020F)a

(0.54% Si)

Retained

Austenite ,c %

46—

45

—

38

25

Seated to and cooled

from the subcriti

cal treatmen

t temperature at 12 C/hr

(22 F/hr).

^Program cooled to

simulate a 150-mm (6-inch) thick plate.

Table4. Hardness of the Experimental Irons after Subcritical Heat Treatment for 5 Hr at Temperatures of 450-550C

(840- 1020F)a.

Fig. 9. Hardness of the experimental irons after heating at 12°C/hr (22° F/hr) to temperatures of 450-550C (840-1020F), holding for 5 hr and cooling at 12°C/hr (22° F/hr).

500 550 4S0 500

TEMPERATURE,^ C

Fig. 10. Hardness of an austenitic-martensitic as-cast 2.9 C-17.5 Cr-1.5 Mo-1 Cu iron after rapidly heating to temperatures above 550C (1020F), holding for various times and air cooling.

Table. 6. High-Stress Abrasive Wear of the Experimental Irons after Tempering at 205C (400F) and after Subcritical Heat Treatment

In Table 7, the high-stress abrasive wear test results for the experimental

irons are compared to those obtained for other ferrous alloys tested previously on i00-/um garnet. With the exception of the pearlitic high-Cr irons, the alloyed white irons exhibit much better abrasion resistance than the quenched-and- tempered 1090 steel and the as-cast and heat-treated austenitic manganese steels. Relatively low abrasion resistance of the pearlitic high-Cr irons serves to reiterate the point that good abrasion resistance can be obtained in high-Cr irons only if the iron has enough hardenability to avoid transformation to pearlite either during mold cooling (as-cast irons) or during cooling following reaustenitizing (heat-treated irons). Among the alloyed white irons, abrasion resistance of the experimental as-cast and subcritically heat-treated irons is comparable to that exhibited by high-Cr-Mo irons which were reaustenitized at 955C {1750F) and cooled at the rate of a 100-mm (4-in.)-thick plate cooling in still air. Furthermore, experimental irons exhibited better abrasion resistance thatn either Ni-Cr iron.Mechanical PropertiesResults of the laboratory tests to determine compressive strength, static and dynamic fracture toughness and impact- bend tensile strength are summarized in Table 8. The two 2.6 C- 20 Cr-2.5 Mo irons were selected for testing and both irons were tested in each of the following conditions:• as-cast• as-cast + 205C (400F) temper• subcritically heat treated to maximum hardness• subcritically heat treated to maximum hardness + 205C

(400F) temper

The subcritical heal treatment was selected on the basis of the earlier experiments.

*

205

(400 F)

Temper

Subcritical Heat Treatment3

Heat

Nominal Composition

No.

Weight Loss, mg

Weight Loss, mg

HV30

100-lim Garnet

180-ym Garnet

HV30

100-Mm Garnet

180-pm Garnet

5670

3C-18Cr-2Mo (0.48Si)

655 26.1 65.6 701

—

49.7

5757

3C-18Cr-2Mo (1.17Si)

613

20.8 60,2. 697

____ 48.8

sSssp-

5813

2.6C-20Cr-2.5Mo (0.54Si)

593

18.4 58.4 665

16.1 50.3

5816

2,6C-20Cr-2.5Mo (1.13Si)

621

22.0 69.2 718

17.2 52.2

3 Treatments selected to produce maximum hardness - see Table 4.

Ultimate compressive strengths of both irons were similar in the as-cast condition and after the various subcritical or tempering treatments. Subcritical treatments produced a significant increase in hardness but only a small increase in compressive strength. Tempering both the as-cast and sub- critically treated irons at 205C (400F) resulted in a small decrease in compressive strength. ^

Static fracture toughness of the irons ranged from 24.0 to 29.0 MPa\/m

(21.8 to 26.4 ksi\/!n.). The 0.54% Si iron generally exhibited slightly better

static fracture toughness than the 1.13% Si iron. The beneficial effect of Si

on fracture toughness, previously observed in as-cast

Table 9. Fracture Toughness of High Chromium-Molybdenum Irons with Austenitic,

Austenitic-Martensitic and Martensitic Matrix Microstructures

Matrix Microstructure

Nominal Composition

SectionSize, mm (in.)

Heat Treatment Hardness, HV30

Fracture Toughness

KIc» MPav^m (ksi/En".)

Austenitic (As-Cast)

3C-18Cr-2Mo 50 (2) 205 C (400 F) - 4 hr 520-565 26.2-34.5 (23.8-31.4)

Austenitic- Martensitic

3C-18Cr-2Mo 100 (4) 205 C (400 F) - 4 hr 613-655 29.1-32.3 (26.5-29.4)

2.6C-20Cr-2.5Mo 150 (6) 205 C (400 F) - 4 hr

475 to 525 C (885 to 975 F) - 4 hr

593-621

665-718

26.5-28.0 (24.1-25.5)

24.0-28.9 (21.8-26.3)

Martensitic (Heat Treated)

3C-18Cr-2Mo 75 (3) 1010 C (1850 F) - 4 hr Air Cool, 205 C (400 F) - 4 hr

760-810 26.9-29.5 (24.5-26.8)

austenitic 18

Cr-2 Mo-1 Cu

irons,'1 was not

evident in

results

obtained for

the

experimental

austenitic-

martensitic irons of this

study.

Subcritical heat

treatments resulted in lower

dynamic fracture toughness

in both irons. Irons in the as-

cast condition or tempered at

205C (400F) exhibited Km

values of 20.6-21.7 MPavm

(18.7-19.7 ksi\An^)

while the_subcritically treated irons had Ku( values of 18.5-19.1 MPa\/m

(16.8-17.4 ksiv/in.).

Type OF alloy

Nominal composition Condition Hardness, hv30

Pin test weight loss, MG (100-UM carnet)

Carbon STEELS Aisi 1020 AISI

1090Cold ROLLED QUENCHED AND TEMPERED

224 735 136 66

Austenitic MN

STEELS6MN-LMO (0.S-0.9C) 7MN-LMO (1.0-1.2C) 12MN-LMO (0.9-1.1C) 12MFI (1.0C)

As-CAST 75-MM (3-IN.) Section SOLUTION TREATE 1 AND QUENCHED AS-CAST 75-MM (3-LN.) Section SOLUTION TREATED ANE QUENCHED

214-253 195-205 190-221 212-217

48-49 56-6069- 7770- 74

PearliticHigh-crIrons

3.2c-15cr 2.6c-20cr Cast 50-MM (2-IN.) Section tempered AT 205c (400 f)

482 398 56 77

Austenitic HIGH CR-KO IRONS

2.9C-17.5CR-L.5MO (VARIOUS CU, NI T MN CONTENTS)

Cast 50-MM (2-IN.) Section TECTPERED AT 205 C (400 F)

470-530 16-23

Austenitic- MARTENSITIC HIGH CR-MO IRONS

2.6C-20CR-2.5MG Cast 150-MM (6-IN.) Section TEMPERED AT 205 C (400 F)Cast 150-MM (6-IN.) Section SUBCRITICALLY HEAT TREATED AT 475 TO 525 C (885 TO 975 F)

593-621

665-718

18-22

16-17

Martensitic HIGH CR-MO IRONS

2.9C-17.5CR-1.5MO (VARIOUS CU, NI 4 MN CONTENTS)

955 C (1750 F), AIR COOL, 205 C (400 F) [50-100 MM (2-4 IN.) Section]

652-786 13-18

Ni-CR IRONS 3. 4C- 3. 9NI- 2. Ocr

3.2C-5.8NI-8.9CR

Cast 25-MM (1-IN.) Section TEMPERED AT 275 C (525 F)Cast 25-MM (1-IN.) Scction CRYOGENICALLY TREATED AT -195 C (-320)

700 820 34 29

Program cooled to simulate a 150-mm (6-in) thick plate.

^Heated to and cooled from the subcritical treatment temperature at 12 C/hr (22 F/hr).

Heat No.

Heat Treatment Hardness, HV30

Ultimate Compressive Strength, MPa (ksi)

Fracture Toughness Impact—Bend Tensile Strength, MPa (ksi)KIc (Static),

MPa>4" (ksi/JET.)

KId (Dynamic),

MPa>4r (ksi/irT.)

5813 (0.54% Si) As-Casta

205 C (400 F) - 4 hr

525 C (97 5 F) - 5 hrb

525 C (977 F) - 5 hrb, 205 C (400 F) - 4 hr

630 593

665647

2720 (395)

2520 (365)

2740 (398)

J.-!■ fri-sxS X-Hi; >

2550 (370)

29.0 (26.4) 28.0

(25.5)

28.9 (26.3)

^fei*,..-,-.... iffiUBfitiErtt

21.1 (19.2) 20.6

(18.7) 18.5 (16.8)

590 (85) 690

(100) 610 (89)

27.3 (24.8) 18.7 (17.0) 600 (87)

5816 (1.13% Si) As-Casta

205 C (400 F) - 4 hr

475 C (885 F) - 5 hrb

475 C (885 F) - 5 hrb, 205 C (400 F) - 4 hr

640 621

718

701

2670 (387)

2650 (384)

2820 (409)

2670 (387)

27.1 (24.7) 26.5

(24.1) 24.0 (21.8)

26.8 (24.4)

21.7 (19.7) 21.1

(19.2) 18.9 (17.2)

19.1 (17.4)

590" . (86)

690 (100) 570

(82)

620 (90)

The best impact-bend tensile strength was obtained in the as-

cast irons which were tempered at 205C (400F). The two irons had

identical impact-bend tensile strength values of 690 MPa (100 ksi)

while the as-cast or subcritically treated irons exhibited lower

impact-bend tensile strengths ranging from 570 to 620 MPa (82 to

90 ksi).

To put these results in perspective, mechanical properties of the

experimental irons were compared to those obtained in previous

investigations of as-cast austenitic and heat-treated martensitic

irons with similar chemical compositions. Figure 11 shows the

compressive strength of various as-cast and heat- treated high-Cr-

Mo irons. In general, experimental austenitic- martensitic irons

exhibited compressive strength values between those exhibited by

austenitic irons with lower hardness and martensitic irons with

higher hardness. As shown in Table 9, experimental irons exhibited

static fracture toughness values which were similar to those

exhibited by austenitic and martensitic irons. Dynamic fracture

toughness values of the experimental austenitic-martensitic irons

are between those exhibited by austenitic (higher Kid) and

martensitic (lower Kid) irons, as illustrated in Table 9.

Subcritically treated experimental irons had Kid values similar to

those exhibited by the martensitic irons.

When used in applications encountering severe repeated impact,

high-Cr-Mo irons must exhibit adequate resistance to spalling. At

present it is not known whether there is a relation between

compressive strength or fracture toughness and spalling resistance

and for this reason it was not possible to compare spalling

resistance of the experimental austenitic- martensitic irons to that

of either austenitic or martensitic irons. Additional research is

underway to determine how mechanical properties or

microstructural variables such as retained austenite and carbide

morphology influence spalling resistance in alloyed white irons and

to evaluate spalling resistance of the experimental austenitic-

martensitic high-Cr-Mo irons.

Summary

Characteristics of as-cast and subcritically heat-

treated high-Cr- Mo irons were evaluated for

irons which were produced asthick-section castings. Compositions selected for study had, sufficient hardenability to avoid forming ferrite-carbide transformation products during the relatively slow mold- cooling rates experienced in 100- to 150-mm

(4- to 6-in.) -thick plates. Microstructure of the as-cast irons contained austenite and martensite and the irons exhibited hardness values of about 650 Vhii 30. Subcritical heat treatment at temperatures of450- 550C (840-1020F) produced an increase in hardness, up to about 720 Vhn 30 and a decrease in the amount of retained austenite. Abrasive wear resistance, compressive strength and fracture toughness were similar to those exhibited by

high-Cr-Mo irons subjected to high-temperature heat treatments.

References1. F. Maratray, R. Usselgio-Nanot, "Transformation Characteristics of Chromium and Chromium-Molybdenum White Irons," Climax Molybdenum Co (1970).2.

R B. Gundlach, "Microstructure, Hardness and Abrasion Resistance of As-Cast 17.5% Chromium White Irons," AFS Transactions, vol 82, p 309-316 (1974).3. D E. Diesburg, F. Borik, "Optimizing Abrasion Resistance and Toughness in Steels and Irons for the Mining Industry," Materials for the Mining Industry, Climax Molybdenum Co, p 15-38 (1974).4. C. Kim, "X-ray Method of Measuring Retained Austenite in White Cast Irons," to be submitted to Journal of Heat Treating for / publication.5 J. Muscara, M. J. Sinnott, "Construction and Evaluation of a Versatile Abrasive Wear Testing Apparatus," Metals Engineering Quarterly, vol 12, no. 2, p 21-32 (May 1972).6. R. S. Jackson, " The Austenite Liquidus Surface and Constitutional Diagram for the Fe-Cr-C Metaslable System," Journal of the Iron and Steel Institute, vol 208, p 163-67 (1970).7. R. Benz, J. F. Elliot J. Chipman, "Thermodynamics of the Carbides in the System Fe-Cr-C," Metallurgical Tiansactions, vol 5, p2235-40. (1974).8. F. Maratray, R. Usseglio-Nanot, "Factors Affecting the Structure of Chromium and Chromium-Molybdenum White Irons," Climax Molybdenum Co, (1970).

9. F. Maratray, A. Poulalion,"Contribution a L'Etudedela Durete Des Structures Martensitiques et de la Retention D'Austenite de son Controle et desa Transformation dans les Alliages Ferreauxa Hautes Teneurs en Carbone et

Fig. 11. Compressive strength as a function of hardness for pearlitic,

martensitic high-chromium irons.

en Chrome," presented at 2nd International Colloquium on Iron Alloys with High Chromium and High Carbon Contents for Tools, Wear Parts and Rolling Mill Rolls, St. Etienne, France (Nov 1977).

The best impact-bend tensile strength was obtained in the as-

cast irons which were tempered at 205C (400F). The two irons had

identical impact-bend tensile strength values of 690 MPa (100 ksi)

while the as-cast or subcriticaiiy treated irons exhibited lower

impact-bend tensile strengths ranging from 570 to 620 MPa (82 to

90 ksi).

To put these results in perspective, mechanical properties of the

experimental irons were compared to those obtained in previous

investigations of as-cast austenitic and heat-treated martensitic

irons with similar chemical compositions. Figure 11 shows the

compressive strength of various as-cast and heat- treated high-Cr-

Mo irons. In general, experimental austenitic- martensitic irons

exhibited compressive strength values between those exhibited by

austenitic irons with lower hardness and martensitic irons with

higher hardness. As shown in Table 9, experimental irons exhibited

static fracture toughness values which were similar to those

exhibited by austenitic and martensitic irons. Dynamic fracture

toughness values of the experimental austenitic-martensitic irons

are between those exhibited by austenitic (higher Kid) and

martensitic (lower Kid) irons, as illustrated in Table 9.

Subcriticaiiy treated experimental irons had Kid values similar to

those exhibited by the martensitic irons.

When used in applications encountering severe repeated impact,

high-Cr-Mo irons must exhibit adequate resistance to spalling. At

present it is not known whether there is a relation between

compressive strength or fracture toughness and spalling resistance

and for this reason it was not possible to compare spalling

resistance of the experimental austenitic- martensitic irons to that

of either austenitic or martensitic irons. Additional research is

underway to determine how mechanical properties or

microstructural variables such as retained austenite and carbide

morphology influence spalling resistance in alloyed white irons and

to evaluate spalling resistance of the experimental austenitic-

martensitic high-Cr-Mo irons.

Summary

Characteristics of as-cast and subcriticaiiy heat-

treated high-Cr- Mo irons were evaluated for irons

which were produced as thick-section castings.

Compositions selected for study had, sufficient

hardenability to avoid forming ferrite-carbide'

transformation products during the relatively slow

mold-

cooling rates experienced in 100- to 150-mm (4- to 6-

in.) -ihick plates. Microstructure of the as-cast irons

contained austenite and martensite and the irons

exhibited hardness values of about 650 Vhn 30.

Subcritical heat treatment at temperatures of 450-

550C (840-1020F) produced an increase in hardness,

up to about 720 Vhn 30 and a decrease in the

amount of retained austenite. Abrasive wear resistance, compressive

strength and fracture toughness were similar to those exhibited by

high-Cr-Mo irons subjected to high-temperature heat treatments.

References1. F. Maratray, R. Usselgio-Nanot, "Transformation Characteristics of Chromium and Chromium-Molybdenum White Irons," Climax Molybdenum Co (1970).2. R B. Gundlach, "Microstructure, Hardnessand Abrasion Resistance of As-Cast 17.5% Chromium White Irons," AFS Transactions, vol 82, p 309-316 (1974).

D E. Diesburg, F. Borik, "Optimizing Abrasion Resistance and Toughness in Steels and Irons for the Mining Industry," Materials for the Mining Industry, Climax Molybdenum Co, p 15-38 (1974).3. C. Kim, "X-ray Method of Measuring Retained Austenite in White Cast Irons," to be submitted to Journal of Heat Treating for publication.5 J. Muscara, M. J. Sinnott, "Construction and Evaluation of a Versatile Abrasive Wear Testing Apparatus," Metals Engineering Quarterly, vol 12, no. 2, p 21-32 (May 1972).6. R. S. Jackson, "The Austenite Liquidus Surface and Constitutional Diagram for the Fe-Cr-C Metastable System," Journal of the Iron and Steel Institute, vol 208, p 163-67 (1970).7. R. Benz, J. F. Elliot J. Chipman, "Thermodynamics of the Carbides in the System Fe-Cr-C," Metallurgical Tiansactions, vol 5, p2235-40. (1974).8. F. Maratray, R. Usseglio-Nanot, "Factors Affecting the Structure of Chromium and Chromium-Molybdenum White Irons," Climax Molybdenum Co, (1970).9. F. Maratray, A. Poulalion,"Contribution a L'Etudedela Durete Des Structures Martensitiques et de la Retention D'Austenite de son Controle et desa Transformation dans les Alliages Ferreauxa Hautes Teneurs en Carbone et en Chrome," presented at 2nd International Colloquium on Iron Alloys with High

Fig. 11. Compressive strength as a function of hardness for pearlitic,

martensitic high-chromium irons.

Chromium and High Carbon Contents for Tools, Wear Parts and Rolling Mill Rolls, St. Etienne, France (Nov 1977).