The Design and Properties of Steel Castings

PART III - Properties

W.J. JACKSON

In Part 111 of this series the properties of steel castings and their implications for design are discussed.

Engineering Properties The effect of carbon content and

heat treatment on the tensile prop- erties of plain carbon steels is illus- trated in Fig. 1. Typical properties available from low-alloy steels are indicated in Fig. 2 (a) and (b); it is apparent from these diagrams that for this class of steel, quenching and tempering can produce higher strengths

and a superior combination of mech- artical properties than normalising and tempering. A summary of minimum specified mechanical properties and typical physical properties is given in Table 1 for some steels to BS 3100. Steels for pressure-containing castings are similar to those in BS 3100, but are covered by BS 1504. A typical application for a high strength steel is shown in Fig. 3 -- how else could such a combination of strength, aesthetic and functional design be achieved, other than by a steel casting?

1000

750

%

Z

"1"

Z

500

Z w p-

250

Fig. 1

ANNEALED - - - - - - NORMALIZED

'~

Table 1 (a) Specified Minimum Mechanical Properties and Physical Properties of some commonly-used Carbon and Low-Alloy Cast Steels

BS 3100 A1 A2 A3 AW2 AW3 Steel Type 0.25%C 0.35%C 0.45%C 0.5%C 0.6%C

Minimum Specified Properties Tensile Strength (Rm)N/mm 2 430 490 Yield Stress (ReL) or 0.2% Proof Stress (RpO.2) 230 260 N/mm 2 Elongation on 5.65x/A % 22 18 Angle of Bend 120 a 90 a Radius of Bend 1.5t ab 1.5tab Charpy V-notch Impact, J 25 a 20 a Brinell Hardness - -

540 620 690

295 325 370

14 12 8

18 c - _

Final Heat Treatment not spec- not spec- not spec- A, N or A, N or ified ified ified NT NT

Representative Physical Properties Specific Gravity

Specific Heat (c) J/Kg C at 29C Thermal Conductivity (k) W/mC

Mean Coefficient of Thermal Expansion (c~) x 106/C 20C-100C

20C-200C

20C-300C

20C-400C

20C-500C

29C-600C

Electrical Resistivity (/9) microhm m at 20C Youngs Modulus (E) N/mm 2

AorN

7.81 A 460 A 50C 100C 44.38 42.7

A N 12.8 12.5

13.1 12.9

13.4 13.3

13.6 13.8

13.8 14.1

14.1 14.5

A 2,290 AorN

A, N or WQT 7.85 A, N or 460 A 50C 100C 43.96 47.7

A N 12.5 12.5

12.7 12.6

13.1 13.4

13.5 13.7

13.9 14.0

14.3 14.4

A 2,288 AN or WQT

203,864-207,725 209,579-211,587

A, N or A, N or A, N or WQT QT QT 7.84 7.9 7.8 7.8 A or WQT A A 460 460 460

50C 100C 50C 43.54 43.12 & 100C

A N A N A 10.8 11.4 12.5 11.8 11.78 WQT 11.7 QT 11.9 12.2 12.2 12.8 12.2 12.36 WQT 12.4 QT 12.4 12.2 12.6 13.2 12.8 13.04 WQT 12.8 QT 12.9 13.4 13.1 13.7 13.2 13.36 WQT 13.2 QT 13.3 13.9 13.5 14.1 13.7 13.74 WQT 13.8 QT 13.8 14.2 13.9 14.4 14.2 14.14 WQT 14.1 QT 14.3

A - 2,354 2,457

A 209,019 A 208, 374 210,815 N 213,205 N 212, 741 214,212 QT 213,749 QT 216,734

Modulus of Rupture N/mm 2 Modulus of Rigidity N/mm 2

Endurance Limit N/mm 2 Notched Un-notched

A A A A 494 544 544 547 A A A A 73,360 79,538 79,538 75,677

N 75 N A 230 208

ANT 179 193 - ANT A 229 259 208

311 MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981

A4 A5 B 1 B 2

1%%Mn 1%Mn C-%Mo 1%Cr-Mo

B3 2%Cr- l%Mo

B5 5%Cr-Mo

BT1 BW2

C- l~r

540 620 460 480

320 370 260 280

16 13 18 17 - - 120 a 120 a _ _ 1.5tab 1.5tab 30 25 20 a 30 a 152-270 d 179-229 d _ 140-212 d

N, NT or N, Ntor QT QT NT NT

540

325

17 120 a 3tab 25 a 156-235 d

NT

620

420

13 90 a 3tab 25 a 179-255 d

NT or QT

69O

495

11

35 f 201-255 d

QT

m

201-255NT d

A, NT or QT

WQT WQT

7.83 7.82 7.86 7.85

460 e 460 e

50C 100C 50C 42.1 42.7 & 100C

7.88

460 e

NT WQT ANT

7.80 7.85 7.8

460 e _ _ NT WQT 50C 100C50C 100C 30.1 30.6 39.7 39.9

WQT WQT NT 13.04 12.4 12.4 11.8 11.8 11.8

12.32 12.8 12.8 12.5 12.3 12.0

13.83 13.3 13.1 12.6 12.6 12.3

14.44 13.9 13.4 13.3 12.9 12.5

14.92 14.6 13.8 13.7 13.4 12.7

16.28 15.0 14.2 14.0 13.8 13.0

NT - - 2,421 2,870 - 3,720 WQT WQT NT 203,015 207,571 205,910 207,100 213, 200 213,980

WQT NT 12.5 12.50

12.7 12.98

13.0 13.26

13.4 13.39

13.9 13.49

14.4 13.53

WQT 2,767 WQT NT 209,580 212,514

WQT 639 - 565 602 WQT 83,930 - 86,480 81,850

NT QT 218 258

334 403 255

NT 653 NT 81,080

WQT 661 WQT 83,400

m

WQT 309

MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981 312

~ o ~,

o~

0 Z E

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Z

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g ~ N ~ N

0 =

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=.

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~ 0 0 0 0 0 0 0 0 ~ O ~ ~ ~ ~ ~ ~ Z

313 MATERIALS IN ENGINEERING, V01.2 , DECEMBER 1981

Legend to Table 1 A and B

A = Annealed. N = Normalised. T = Tempered. Q = Quenched. SR = Stress Relieved. WQ = Water Quenched. OQ = Oil Quenched. H = Air Hardened. a Either Charpy or bend test may be specified. b t= Thickness of test piece c Impact test is mandoatory only if specified by purchaser. d BrineU hardness is only required if requested by the purchaser and stated on the order. e 25C. f Where due to design of casting, it is not practicable to liquid quench the casting it may not be possible to achieve the specified impact

properties. In such cases the values to be obtained should be agreed between the manufacturer and purchaser at the time of enquiry or order.

g -60C. h Not applicable to free machining steel. k Free machining specification in brackets. m Provision is made for supply of this steel with specified impact at low temperatures (-196C) if stated on order, if agreed between

manufacturere and purchaser.

1 1oc~o zX.~l

\

I 800

I I / I 1ooo 12oo I aoo 16oo

T~NS~LE STRENGTH N mm 2

Fig. 2(b) Tensile properties of low alloy cast steels (quenched and tempered).

test results are given in Table 21 . Machined castings for use in marine and slow speed diesel engines are shown in Fig. 5.

The presence of a notch reduces the endurance limit of both cast and wrought steels. Apart from notches, surface condition has an important bearing on fatigue properties. For example, when the casting surface remains, the endurance limit is lowered by 30%, compared with a polished surface. The as-cast surface, however,

gives fatigue properties superior to as- roiled or as-forged surfaces.

For design purposes, as mentioned

Fig. 3 Bruce Anchor, cast in high strength steel (Courtesy of Bruce Anchor Ltd.).

L Plain carbon , annea led

Unnotched Notchea

Wrought O Cast C3

2OO I , I J~ l l

lO a lOs lO 6 lo ~ CYCLES TO FA ILURE

Fig. 4(a) S-N curves for plain cast steel (annealed) and com- parison with wrought steel.

earlier in the text, it is important to know crack initiation and crack propagation data, which can be used in a truly quantitative manner for

, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o,ou0h, c,,, :~ . . . . . . . . . . . . . oo ."

i i i i i i i i I i i i i I i i i J i i i i i l J l J lO 4 lO 5 to 6 , , i

CYCLES T(] FA ILUHF

2~

Fig. 4(b) S-N curves for plain carbon cast steel, normalised and tempered and comparison with wrought steel.

prediction of service life. Recently generated data on these parameters are summarised in Table 3 2, 3, 4 Fracture toughness data, mainly gen- erated by SCRATA, that can be used in conjunction with the fatigue prop- agation data, are summarised in Table 4.

Factors Affecting Mechanical Properties

The element in cast steel that in- fluences properties more than any other is carbon. In general, increasing the carbon content will raise the hard- ness, yield stength, and tensile strength of a steel, while simultaneously ductil ity is decreased. For a steel of a given carbon content, heat treatment can produce a wide range of properties, and this range becomes wider as carbon content increases.

Alloying elements exert their influ- ence on properties by altering the form and dispersion of carbon in the steel. They allow for greater hardness and strength, without the loss in ductil ity

MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981 314

Table 2 Fatigue Properties of Typical Cast Steels in Different Conditions of Heat Treatment with Notched and Un-Notched Test Pieces

Steel Heat Tensile Yield Reduction Specification Treatment* Strength Strength of Area, % and Type N/mm N/mm

Elongation L=4D,%

BS 3100 A3 A900C 576 340 46.7 0.4%C N900 o, T650C 649 386 52.2

BS 3100 A5 N900 T425C 669 437 58.5 11/2% Mn N900 , T650C 684 428 58.5

Q845 , T620C 843 731 55.6

BS 3100 BT1 N900 , T650C 777 596 43.1 Cr-Mo Q845 , T635C 1010 903 35.8

BS 3100 BT2 N900 , T650C 761 590 53.7 Ni-Cr-Mo Q845 v, T650C 948 865 34.5

BS 3100 BT3 N900 , T605C 871 692 50.2 Ni-Cr-Mo Q830 , T550C 1160 1075 17.8

27.5 24.5

26.0 24.0 20.5

18.0 14.0

19.0 14.8

14.5 9.5

Steel Brinell Endurance Specification Hardness Limit~ and Type Number N/mm

Un- Notched Notched

BS 3100 A3 156 229 179 0.4%C 187 259 193

BS 3100 A5 201 287 215 1% Mn 201 333 218

269 403 258

BS 3100 BT1 223 354 230 Cr-Mo 311 423 279

BS 3100 BT2 223 372 229 Ni-Cr-Mo 286 448 266

BS 3100 BT3 262 434 241 Ni-Cr-Mo 375 534 332

Endurance Ratio

Un- Notched Notched

Notch Sensitivity factor, q

0.40 0.31 0.23 0.40 0.30 0.29

0.43 0.32 0.28 0.49 0.32 0.44 0.48 0.31 0.48

0.45 0.30 0.45 0.42 0.28 0.43

0.49 0.30 0.53 0.47 0.28 0.57

0.50 0.28 0.68 0.46 0.29 0.51

* A = Annealed; N = Normalised; Q = Liquid Quenched; T = Tempered.

which would occur if a similar hardness were obtained by increasing the carbon content. In the main, their purpose is to increase hardenability in thicker sections. This is done by altering the thermal transformation characteristics of the steel, with the result that more complete hardening can take place in thicker sections. The depth-of-harden- ing capabilities of a steel are measured by the Jominy end-quench test, typical results being given in Fig. 6 for a 1% Mn-Mo steel, and a similar steel with a boron addition s . The top curve for the steel containing boron shows that high hardness is maintained at much greater depths in the testpiece.

Alloying additions in excess of those required for minimum hardenability can result in some deterioration in properties. This is possibly due to increased segregation effects and the presence of retained austenite 6 .

The section size or mass of any metal part will affect mechanical properties by virtue of its influence on freezing rate and cooling rate after heat treatment. In steel castings, the effect of mass on mechanical properties is due to such factors as segregation, non-metallic inclusions, micro- structure, and density. In studies on the effect of mass, it has been found that, with increasing mass, there was a decrease in strength and ductility measured at the centre of a section, a decrease in density and carbon con- tent, and a slight decrease in notched- bar impact strength 7 .

The effect of hardenability on the mechanical properties of three section sizes of a Ni-Cr-Mo steel is shown in Fig. 7. The ductilities of the 30, 75 and 150mm sections do not differ greatly from the normalised and tempered to the quenched and temp-

ered conditions. However, the smaller sections exhibit greater strengths and uniformity than the larger pieces. The bigger difference in strength from edge to centre in the quenched steel is merely due to the outer layers having been fully hardened by quenching. The important point is that the prop- erties at the centre of each section have been improved by quenching and tempering, and the yield/tensile strength ratios are considerably higher.

The fatigue test results in Fig. 8 show that, for a given steel and section size, the fatigue properties also tend to be lower in the centre of the section. Since tensile strength tends to follow a similar pattern, the change in endurance ratio is not large. Among factors contributing to the slight decrease in endurance ratio are an increase in grain size, an increase in inclusion size, and microstructural irthomogeneity associated with inter-dendritic segre- gation. This slight reduction in endur- ance ratio is probably of negligible importance with regard to the perform- ance of the section as a whole in bending fatigue, as the outer layers invariably are the most highly stressed. A similar decrease in endurance ratio at the centre of a heavy section is also exhibited by wrought steels.

Nevertheless, reasonable uniformity of properties can be obtained in castings of fairly complex configur- ation, provided that they are properly designed from the foundry point of view. This mainly involves running and feeding considerations, but heat treatment also plays a part.

Low Temperature Properties Heat treatment, by its effect on

microstructure, has a major influence on impact strength and the effect is of particular importance with ferritic steels for low temperature service. This is clearly illustrated by the results of annealing, normalising, and annealing plus normalising, for a low- carbon steel shown in Fig. 9 s .

The transition temperatures of hard- ened a, ~l tempered low- and medium- alloy steels are markedly influenced by the tempering temperature used. In general, the effect of decreasing the tempering temperature (i.e. raising the hardness and tensile strength) is to raise the impact and fracture appear- ance transition temperatures. In addi- tion, the maximum impact values at the higher testing temperatures are lowered by decreasing the tempering

315 MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981

Steel Specification and Type

BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3101/A2 0.3%C BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3100 A2 0.3%C BS 3100 A4 1%M n BS 3100 B7 Cr-Mo-V

BS 3100 BT2 Ni-Cr-Mo

BS 3100 A4 1% Mn BS 31/)0 B7 Cr-Mo-V

BS 3100 BT 1 Mn-Ni-Cr-Mo

BS 3100 BTI 1% Mn-M~

BS 3100 BT2 Ni-Cr-Mo

BS 3100 AW3 Cr-C

Table 3 Fatigue Crack Initiation and Propagation Data for Cast Steels

Test Block Type 0.2% Heat Treatment* and/or Size mm Proof Stress

N]mm 2

R value A Ko min. stress MN/m J/ 2

max. stress n c m B

A 125 x 32 x 300 587 0 9

A 200x 100 x 450 5541o576 0 10

A 200 x 200 x 201/ 542 to 574 0 10

N 125 x 32 x 3110 619 0 10

N 200 x 100 x 450 588 to 604 0

N 200 x 200 x 200 565 to 608 0

A 125 x 32 x 3011 587 1).5 4

A 200 x 1011 x 4511 554 to 576 0.5

A 2110 x 200 x 2011 542 1o 574 0.5

N 125x32x300 619 I).5 6

N 200 x 100 x 451) 588 to 604 0.5

N 200 x 200 x 20(1 565 to 608 0.5 A 960C, N 960C, 2011 x 1110 x 4511 411 0 9.9 A 960C, N 975C, 21111 x 100 x 4511 545 11 I 1.2 T 700C A 900C, OQ 900C, 21)(I x 100 x 4511 774 Y 640C A 960C, N 960C 31111 x 1011 x 132 430 A 950C, N 980C, 31111 x 100 x 132 5011 T 680C SR 680C, WQ 930C, 31111 x 100 x 132 7411 T 600C and wo A 900C, WQ 900C, 31111 x 1011 x 132 6611 T 500C and WQ Re-temper- ed 610(" and wo A 900"C, OQ 880C, 30(I x 100 x 132 8511 T 640C

A 900C 3011 x 100 x 132 360

2.11 8.03 X 10- 8 -10.1

2.90 5 X 10 -9 -18.65

3.32 1.26X 10- 8 -24.04

2.01 9.9 X 10 -8 -9.67

2.64 1.1 X 10 -8 -18.25

3.06 2.29 x 10 -9 -17.21

2.11 8.03 x 10- 8 -9.93

2.90 5 x 10 .9 -8.02

3.32 1.26 x 10- 8 -8.26

2.1)1 9.9 x 10- 8 -12.29

2.64 1.1 x 10- 8 -9.11

3.06 2.29 x 10 -9 -4.98

2.29 3.59x 10 -8 -9.93

2.13 5.27 x 10- 8 -16.35

3.43 x 1038

3.85 x 1067

6.09 x 1085

1.58 x 1037

4.45 x 1066

2.38 x 1063

8.98 x 1035

6.02 x 1029

6.3 x 1030

3.59 x 10 `*2

5.38 x 1033

1.63 X 1021

2.42 X 1038

7.78 X 1060

0 11.1 1.49 6.26 x t0- 7 -15.79 1.34 x 1059

2.07 10 -7 -3.53 101515

2.11 10-6. 5 -4.02 10166

10-5.7 -4.76 10197

1.94 10- 7.t -4.25 101765

1.80 10-6.7 -5.0 1020.8

2.09 10 -6.9 -3.17 1013"6

* A = Annealed: N = Normaliscd; SR = Stress Relict" Annealed; WQ = Water Quenched" OQ = Oil Quenched: T = Tempered

temperature. These effects are illust- rated in Fig. 10 for a 2% Ni-Cr-Mo steel tempered to three hardness levels s .

Austenitic stainless steels usually retain a high toughness at very low temperatures, although impact strength can rapidly decrease as the temperature is lowered. A fully austenitic structure or a low magnetic permeability is not necessarily a guarantee of good low- temperature properties, although large amounts of delta ferrrite in 18/8

types of steel will lower impact properties. Other factors, e.g., grain size or distribution of carbides, can also have an effect. Non-stabilised 18/8 type steels are markedly superior to the titanium and niobium-stabilised steels. In general, for optimum low- temperature properties, carbon and stabilising additions should be kept to a very low level.

To summarise the applicability of the various types of steels, it may generally be said that low-carbon

cast steel is satisfactory at temper- atures down to 40C or -50C, low- and medium-alloy steels at temper- atures down to -60C; at lower temp- eratures, austenitic steels are required.

Elevated Temperature Properties Steel castings are widely used in

environments where the temperature is above that of normal room temper- ature. In cases where the temperature is not very greatly higher, no distinction is made between the properties at that

MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981 316

Material Specification and Type

BS 3100 B2 Plain Carbon

BS 3100 A4 1V2%Mn

BS 3100 A4 1%M n

BS 3100 BTI Mn-Ni-Cr-Mo

BS 3100 BT2 Ni-Cr-Mo

BS 3100 BT2 Ni-Cr-Mo

BS 3100 BT2 Ni-Cr-Mo

BS 3100 BT2 1%Mn-Mo

BS 3100 BW2 1% Cr-C

BS 3100 BW2 1% Cr-C

BS 3100 B7 %Cr-%Mo-%V

BS 3100 B7 %Cr-%Mo-%V

BS 3100 B7 %Cr-%Mo-%V

BS 3100 AW3 0.5%C

BS 3100 410C21 13%Cr

Table 4 Room Temperature Fracture Toughness Data for a Variety of Cast Steels

0 .2~ Proof KIC 3/~ COD**

Heat Treatment* Section Stress MN/m ~; tSi or ~c Size mm N/mm 2 (N/mm 3/2) mm

A 950C 25 323

A 960C , N 960C 25 427

A 960C , 50 N 960C 20 412

SR 680~ WQ 930C, 25 742 72 T 600C and WQ (2275)

A 900C , OQ 880C 25 853 T 640C

A 950C , 40 750 to 66 to 96 SR 600C, 820 (2086 to N 920C , 3034) T 600C 20

A 900C , 30 715 to 85 OQ 880C, 740 (2686) T 640C

A 930t)C , 30 640 t() WQ 890C, 758 T 650C and WQ

A 920C , N 870C , 60 419 59 T 635C (1860)

A 920C , N 870C , 25 46(1 48 T 635C (1517)

A 950C , N 980C , 25 504 46 T 680C and (1454) T 700C

A 1000C, 60 367 It) 54 to 69 N 1000C, 550 (1706 to T 680C 2180)

A 960C , 50 520 to 52 to 71 N 975C , 585 (1643 Io

2244) T 700C 20

A 900C 25 360

0.04 to 0.08

0.18 to 0.29

0.129 to 0.173 0.095 to 0.155

0.018 to 0.026

0.026 to 0.050

0.021 to 0.11

0.058 to 0.07

0.066 to 0.117

0.057 to 0.133

0.02

0.02

0.01

0.016 to 0.03

0.024 to 0.07

0.032 to 0.05

0.05 to 0.09

A 1050C, 60 366 to N 1050C, 446 T 680C

0.04 to 0.08

K~C/Oy m

0.097

0.084 to 0.122

0.1

0.14

0.1

0.09

0.11 to 0.15

0.09 to 0.12

* A = Annealed; N = Normaliscd; SR = Stress Relief Anncalcd; WQ = Water Quenched; OQ = Oil Quenched; T =Tcmpcrcd. ** The nomenclature for COD measurements varies according to the behaviour of the test piece. An explanation is

containcd in BS 5762.

temperature and room temperature properties. However, at temperatures of the order of 300C, the strength properties deteriorate, as shown for various steels in Fig. 11. While it is obviously important to know the instantaneous or short-time strength properties of a steel at its operating temperature, creep properties are frequently more important for design purposes. This is particularly true in the case of engineering components operating at temperatures up to about

750C, such as valve parts, nozzle segments, turbine housings, impellers, and accumulators. At temperatures ranging up to 750C, oxidation resistance is of secondary importance to creep resistance but, as operating temperatures exceed this order of temperature, the order of importance is reversed, and here furnace and kiln parts may be quoted as examples. For any given instance it may be important to know the short-time, long-time, and scaling properties of a

cast steel. An example is given of a stator ring (Fig. 12(a)) used in the Rolls Royce Olympus engine (Fig. 12 (b)).

For many applications the design life of the component is much longer than a practical laboratory test and it becomes necessary to extrapolate the experimental data. There are several methods for data extrapolation, the most common being based upon time- temperature parameters, which enable a master curve to be constructed from

317 MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981

Fig. 5 Selection of steel castings for the marine industry. (Courtesy of British Steel Corporation).

which stress rupture properties can be determined at temperatures other than the original test temperatures. There are several ways of presenting the stress rupture data, one of the most common being plots of stress vs temperature for lines of constant rupture life. These curves are fre- quently plotted on the same diagram with curves showing the proof stress results from short-time tensile tests, thus giving a complete description of the mechanical properties over wide temperature ranges. A typical graph of this type for a 1% Cr-Mo steel is shown in Fig. 13.

Although the tensile strength provides useful information, the para- meter used in designing components is the proof stress. The proof stress values in current use are the 0.2% for carbon and low alloy (ferritic) steels,

and the 1.0% for austenitic steels. The designer is interested in the minimum

expected value, rather than average values and in recent years an inter- nationally agreed method has been evolved for deriving the former values from test data. Many standards how- ever still contain elevated temperature proof stress minima based upon the wide experience of the steel industry, and this sometimes leads to con- fusion, as to the design safety factors to be applied. Typical minimum 0.2% proof stress properties for 2% Cr-Mo and % Cr-% Mo-% V low alloy steels are given in Figs. 14 and 15 together with average tensile properties.

Magnetic Properties Because of their excellent combin-

ation of strength and suitable magnetic properties, steel castings are very widely used in electrical plant and machinery. Examples of the use of high-permeability steel castings include electromagnetic clutches and brakes, yokes for electromagnets, and stator housings and armatures for electric motors and dynamos.

To obtain high permeability, the carbon content of the steel should be very low. However, for many applic- ations in electrical engineering, strength is an additional requirement of some importance. Consequently, in BS 3100 there are two steels, AM1 having a lower carbon content and better magnetic properties than AM2 which in turn has the better strength. The effect of carbon content is typified in Fig. 169 . Manganese acts in the same way as carbon in impairing mag- netic properties, and it is for this reason that the maximum amount permitted in

10 800

700

> 600 I

500 ~"~., g , ,~ 400 "r"

300

200

20 30 40 b() 60 ,,.n

~'~. 172'/,, Mn Mo

, I , , I , , I . . I , I , , i . . I , . I , = I , , I , , I , , I , , I , ,

3 6 9 12 15 18 21 24 27 30 33 36 39 42

DISTANCE FROM QUENCHED END OF JOMINY BAR, 1/16in.

Fig. 6 Jominy end-quench hardenability curves for lY2%Mn-Mo steels with and without boron.

MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981 318

Ni-Cr-Mo STEEL, N & T

30mm(11/.in 1 TENSILE STRENG'~H !150mm (6in) 100(] O SECTION~ rl 7~mm(3in) SECTION A SECTION

I J 40C ~

50

40-

30- I

~ 20- 0.

10 40

I

30- Z

'-30 mm( 1v, dn) ,_ 75 mm(3 in )

-I

800

% E 600 Z

~ 600

150 mm (6in)

Fig. 7(a) Distribution of tensile properties across a 150mm (6 in.) section of Ni-Cr-Mo cast steel, after normalising and tempering.

these steels is U.5 per cent I o When selecting the grade required

for a particular application, a com- promise must be made between these opposing factors. If, however, a material of very much higher strength is required, the carbon content may be raised or alloying additions may be made. Of the usual alloying elements, carbon is preferred since it gives the most favourable relationship between gain in yield strength and loss in magnetic induction 11.

Silicon and residual elements (S, P, Cr, Ni, Mo, Cu, Pb, Sn, V) have only small effects upon magnetic properties, and are insignificant when the steel is of normal commercial carbon content

(e.g. less than 0.25%) 1o,12,13 Aluminium, which is very necessary for deoxidation of low carbon steels also has no significantly adverse effect upon magnetic properties, when used in normal amounts (e.g. not more than 0.2% added weight) 12 . Non- metallic inclusions, when small and evenly distributed, have no signifi- cant effect on magnetic properties 14.

In applications where there is an alternative magnetic field, it is import- ant to know the hysteresis curve of the steel, as wide hysteresis loops represent considerable energy loss. For carbon contents between 0.10 and 0.34%, hysteresis loss varies from 9,600 to 11,600 ergs/cc/cycle for a

flux density of 1.9 tesla. For appli- cations involving intermittently applied magnetic fields, such as in magnetic brakes and clutches, the residual magnetism should be low 1 s

Designers of electrical apparatus sometimes require non-magnetic cast steels. For this purpose, cast austenitic manganese steel is excellent, as not only is it virtually non-magnetic with a permeability of 1.002 to 1.003, but also it is able to withstand severe wear 16. Where improved machinability on non-magnetic steel is required, the composition may be modified by lowering the carbon content to 0.2%, lowering the manganese content to 10.0%, and adding approximately 7.0% of nickel.

For other purposes, non-magnetic corrosion-resisting steels may be re- quired. Austenitic chromium-nickel steel is occasionally found to be slightly magnetic, whether in worked or cast form. This is because of the presence of a small amount of ferrite, resulting from a particular balance of alloying elements 17.

Section thickness can also in- fluence magnetic properties, as il- lustrated by the data I 8 in Table 5. With regard to the general corrosion resistance of 18/8 type steel at normal temperatures, magnetic properties have no relation to the applicability of the casting. Furthermore, non-magnetism is not necessarily a criterion when considering the possibility of em- brittlement at elevated temperatures by sigma phase. Consequently, very low magnetic properties of steel castings need only be a requirement in electrical applications where non- magnetic material is essential.

Wear-Resisting Properties The replacement of steel parts

that wear in service can be an expensive item and may form a major part of the cost of the process being carried out. Apart from the actual cost of replacements, down-time on the machine can represent a considerable loss.

Resistance to abrasion by non- metallic materials is a function not only of the steel, but also of the environmental conditions. Three general types of wear have been recognised, i.e. gouging abrasion, grinding or high-stress abrasion, and scratching or low-stress abrasion. In any particular service, more than one of these types of wear may operate,

319 MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981

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Ni-Cr-Mo STEEL, WQ & T but it is usually possible to isolate the

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dominant type. Gouging abrasion is actually a

cutting process that occurs when rocks or other lump materials cut into a wearing surface. It can occur at high or low velocities, and can be accomp- anied by severe impact, which can be the main factor deciding the selection of a suitable wear-resistant material. Grinding or high-stress abrasion occurs when two surfaces rub together in the

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presence of an abrasive material, e.g., when gritty materials are crushed between the two surfaces. While overall loads may be low, the stress on individual areas is very high. Scratching or low-stress abrasion or erosion occurs when loose particles move freely on the wearing surface. In this type of abrasion, impact forces are low, so that relatively brittle materials can be used.

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Fig. 7(b) Distribution of tensile properties across a 150mm (6 in.) section of Ni-Cr-Mo cast steel, after quenching and tempering.

Table 5 Magnetic Permeability of Thick and Thin Sections of 18/8 Steel Castings in Relation to Ferrite Content

Thin Section, 20ram Thick Section, 75mm

Magnetic Ferrite Magnetic Ferrite Permability (%) Permeabifity (%)

1.005 0 1.005 0 1.010 0 1.025 0.4 1.084 0.7 1.105 2.3 1.150 1.8 1.615 7.5 1,316 3.2 1.665 7.0 1.659 8.0 2.315 I0.0 2.829 16.2 3,753 19.0

MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981 320

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. . . . NORMALfSED

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Magnetisation curves for 0.10% and 0.34% carbon steels in the annealed and normalised conditions.

323 MATERIALS IN ENGINEERING, Vol. 2, DECEMBER 1981