05 Heattreatment

8/2/2019 05 Heattreatment

1/39

Heat treatment


2/39

Influence of alloyingelement additions oneutectoid temperatureand eutectoid carboncontent.


3/39

Hardenability Concepts

The goal of heat treatment ofsteel is very often to attain asatisfactory hardness.

The important microstructural

phase is then normallymartensite, which is the hardestconstituent in low-alloy steels.

The hardness of martensite isprimarily dependent on itscarbon content as is shown in

If the microstructure is not fullymartensitic, its hardness islower.

Relationship between hardness, carboncontent, and amount of martensite


4/39

Grossmann Hardenability Test

A number of cylindrical steel bars of different diametersare hardened in a given cooling medium.

By means of metallographic examination, the bar thathas 50% martensite at its center is singled out and the

diameter of this bar is designated as the critical diameter(D0).

This D0 value is valid for the particular cooling mediumused as well as its cooling intensity.

An ideal cooling situation is when the surface of the testbar is immediately cooled to ambient temperature, thatis, an infinite cooling rate at the surface.


5/39

Jominy End-Quench Test

A round bar specimen that is 100 mm (4 in.) in length and 25 mm (1 in.) in diameter isused.

The specimen is heated to the austenitizing temperature of the steel with a holdingtime of 20 min.

One endface of the specimen is quenched by spraying it with a jet of water. This causes the rate of cooling to decrease progressively from the quenched end

along the length of the bar.

When it is cool, two diametrically opposite flats that are 0.4 mm (0.015 in.) deep andparallel to the axis of the bar, are ground and the hardness is measured along theflats.

The hardness values are plotted on a diagram at specified intervals from thequenched end.

To get good reproducibility, the time and temperature of austenitizing, the grinding ofthe flats to avoid burning, and the placement of the specimen in the hardness tester

should be carefully controlled. It is also important to protect the specimen against decarburization. Less critical are the water temperature, the diameter of the spraying nozzle, the

height of the water jet, and the time to move the specimen from the furnace to thefixture.


6/39

Hardening procedure for Jominyend-quench specimen

Reproducibility of Jominy testby nine laboratories on oneheat of 4068 steel


7/39

The rate of cooling at different distances fromthe quenched end is approximately independentof the steel used because the thermal

conductivity and heat capacity of hardenablesteels do not vary very much and the heattransfer at the cooled end is steel independent.

Therefore, the Jominy bar presents a range of

cooling curves that can be used to estimate aCCT diagram.


8/39

Relationshipbetween IT, CCT,and Jominy

Curves

Relationship ofCCT (heavy lines)and IT (light lines)

diagrams ofeutectoid steel. Four cooling rates

from differentpositions on a

Jominy end-quench specimenare superimposedon the CCTdiagram


9/39

Normalizing

Heat-treating process that is often consideredfrom both thermal and microstructuralstandpoints.

In the thermal sense, normalizing is anaustenitizing heating cycle followed by cooling instill or slightly agitated air.

Typically, the work is heated to a temperatureabout 55 C (100 F) above the upper criticalline of the iron - iron carbide phase diagram


10/39

STRESS-RELIEF HEAT TREATING

used to relieve stresses that remain locked in a structureas a consequence of a manufacturing sequence.

This definition separates stress-relief heat treating frompostweld heat treating in that the goal of postweld heattreating is to provide, in addition to the relief of residualstresses, some preferred metallurgical structure orproperties

For example, most ferritic weldments are given postweldheat treatment to improve the fracture toughness of theheat-affected zones (HAZ).

Moreover, austenitic and nonferrous alloys are frequentlypostweld heat treated to improve resistance toenvironmental damage.


11/39

Stress-relief heat treating is the uniform heating of astructure, or portion thereof, to a suitable temperaturebelow the transformation range (Ac1 for ferritic steels),holding at this temperature for a predetermined period oftime, followed by uniform cooling

Stress-relief heat treating can reduce distortion and highstresses from welding that can affect serviceperformance.

The presence of residual stresses can lead to stress-corrosion cracking (SCC) near welds and in regions of a

component that has been cold strained duringprocessing.

Furthermore, cold strain per se can produce a reductionin creep strength at elevated temperatures.


12/39

Sources of Residual Stress

Bending

Quenching

GrindingWelding

Examples of the causes of residualstresses: (a) Thermal distortion in a

structure due to heating by solar radiation.(b) Residual stressesdue to welding. (c) Residual stresses dueto grinding.


13/39

Partial iron-iron carbide phase diagram showingtypical normalizing range for plain carbon steels


14/39

Comparison of time-temperature cycles for normalizing and full annealing. The slower cooling of annealing results in higher temperature

transformation to ferrite and pearlite and coarser microstructures than doesnormalizing.


15/39

The purpose of normalizing

Normalization may increase or decrease the strength and hardnessof a given steel in a given product form, depending on the thermaland mechanical history of the product.

Actually, the functions of normalizing may overlap with or beconfused with those of annealing, hardening, and stress relieving.

Improved machinability, grain-structure refinement, homogenization,and modification of residual stresses are among the reasonsnormalizing is done.

Homogenization of castings by normalizing may be done in order tobreak up or refine the dendritic structure and facilitate a more evenresponse to subsequent hardening.

Similarly, for wrought products, normalization can help reducebanded grain structure due to hot rolling, as well as large grain sizeor mixed large and small grain size due to forging practice.


16/39

ANNEALING

a generic term denoting a treatment that consists ofheating to and holding at a suitable temperature followedby cooling at an appropriate rate, primarily for thesoftening of metallic materials.

Generally, in plain carbon steels, annealing produces aferrite-pearlite microstructure

Steels may be annealed to facilitate cold working ormachining, to improve mechanical or electricalproperties, or to promote dimensional stability. The

choice of an annealing treatment that will provide anadequate combination of such properties at minimumexpense often involves a compromise.


17/39

A fully annealed 1040 steel showing a ferrite-pearlite microstructure. Etched in 4% picral plus2% nital.


18/39

Critical Temperatures

The critical temperatures that must beconsidered in discussing annealing of steel arethose that define the onset and completion of thetransformation to or from austenite.

For a given steel, the critical temperaturesdepend on whether the steel is being heated orcooled.

Critical temperatures for the start and completion

of the transformation to austenite during heatingare denoted, respectively, by Ac1 and Ac3 forhypoeutectoid steels and by Ac1 and Accm forhypereutectoid steels.


19/39


20/39

Subcritical Annealing

Subcritical annealing does not involve formation ofaustenite.

The prior condition of the steel is modified by suchthermally activated processes as recovery,recrystallization, grain growth, and agglomeration ofcarbides.

In as-rolled or forged hypoeutectoid steels containingferrite and pearlie, subcritical annealing can adjust thehardnesses of both constituents, but excessively longtimes at temperature may be required for substantial

softening. The subcritical treatment is most effective when applied

to hardened or cold-worked steels, which recrystallizereadily to form new ferrite grains


21/39

lntercritical Annealing

Austenite begins to form when the temperature of the steel exceeds A1. The solubility of carbon increases abruptly (nearly 1%) near the A1

temperature. In hypoeutectoid steels, the equilibrium structure in the intercritical range

between A1 and A3 consists of ferrite and austenite, and above A3 thestructure becomes completely austenitic. However, the equilibrium mixture

of ferrite and austenite is not achieved instantaneously. In hypereutectoid steels, carbide and austenite coexist in the intercritical

range between A1 and Acm; and the homogeneity of the austenite dependson time and temperature.

The degree of homogeneity in the structure at the austenitizing temperatureis an important consideration in the development of annealed structures andproperties.

The more homogeneous structures developed at higher austenitizingtemperatures tend to promote lamellar carbide structures on cooling,whereas lower austenitizing temperatures in the intercritical range result inless homogeneous austenite, which promotes formation of spheroidalcarbides.


22/39

Austenite formed when steel is heated above the A1 temperaturetransforms back to ferrite and carbide when the steel is slowly cooled belowA1.

The rate of austenite decomposition and the tendency of the carbidestructure to be either lamellar or spheroidal depend largely on thetemperature of transformation.

If the austenite transforms just below A1, it will decompose slowly.

The product then may contain relatively coarse spheroidal carbides orcoarse lamellar pearlite, depending on the composition of the steel and theaustenitizing temperature.

This product tends to be very soft. However, the low rate of transformation at temperatures just below A1

necessitates long holding times in isothermal treatments, or very slow

cooling rates in continuous cooling, if maximum softness is desired. Isothermal treatments are more efficient than slow continuous cooling in

terms of achieving desired structures and softness in the minimum amountof time.


23/39

Supercritical or Full Annealing

A common annealing practice is to heat hypoeutectoidsteels above the upper critical temperature (A3) to attainfull austenitization.

The process is called full annealing. In hypoeutectoid

steels (under 0.77% C), supercritical annealing (that is,above the A3 temperature) takes place in the austeniteregion (the steel is fully austenitic at the annealingtemperature).

However, in hypereutectoid steels (above 0.77% C), the

annealing takes place above the A1 temperature, whichis the dual-phase austenite-cementite region


24/39

The iron-carbon binary phase diagram showingregion of temperatures for full annealing


25/39

Hypereutectoid steels can be made extremely soft byholding for long periods of time at the austenitizingtemperature.

Although the time at the austenitizing temperature mayhave only a small effect on actual hardnesses (such as a

change from 241 to 229 HB), its effect on machinabilityor cold-forming properties may be appreciable.

Long-term austenitizing is effective in hypereutectoidsteels because it produces agglomeration of residualcarbides in the austenite.

Coarser carbides promote a softer final product. Inlower-carbon steels, carbides are unstable attemperatures above A1 and tend to dissolve in theaustenite, although the dissolution may be slow.


26/39

QUENCHING

The process of rapidly cooling metal partsfrom the austenitizing or solution treatingtemperature,typically from within the range

of 815 to 870 C (1500 to 1600 F) forsteel.


27/39

The liquid quenchants

Oil that may contain a variety of additives

Water

Aqueous polymer solutions

Water that may contain salt or causticadditives

gaseous quenchants are inert gases

including helium, argon, and nitrogenThese quenchants are sometimes usedafter austenitizing in a vacuum.


28/39

Quenching Process

Direct quenching

Time quenching

Selective quenchingSpray quenching

Fog quenching

Interrupted quenching


29/39

Direct quenching refers to quenching directly from the austenitizing temperature andis by far the most widely used practice. The term direct quenching is used todifferentiate this type of cycle from more indirect practices which might involvecarburizing, slow cooling, reheating, followed by quenching.

Time quenching is used when the cooling rate of the part being quenched needs tobe abruptly changed during the cooling cycle. The change in cooling rate may consistof either an increase or a decrease in the cooling rate depending on which is neededto attain desired results. The usual practice is to lower the temperature of the part byquenching in a medium with high heat removal characteristics (for example, water)until the part has cooled below the nose of the timetemperature-transformation (TTT)curve, and then to transfer the part to a second medium (for example, oil), so that itcools more slowly through the martensite formation range. In some applications, thesecond medium may be air or an inert gas. Time quenching is most often used tominimize distortion, cracking, and dimensional changes.

Selective quenching is used when it is desirable for certain areas of a part to berelatively unaffected by the quenching medium. This can be accomplished byinsulating an area to be more slowly cooled so the quenchant contacts only thoseareas of the part that are to be rapidly cooled.


30/39

Spray quenching involves directing high-pressure streams of quenchingliquid onto areas of the workpiece where higher cooling rates are desired.The cooling rate is faster because the quenchant droplets formed by thehigh-intensity spray impact the part surface and remove heat veryeffectively. However, low-pressure spraying, in effect a flood-type flow, ispreferred with certain polymer quenchants.

Fog quenching utilizes a fine fog or mist of liquid droplets in a gas carrieras the cooling agent. Although similar to spray quenching, fog quenchingproduces lower cooling rates because of the relatively low liquid content ofthe stream.

Interrupted quenching refers to the rapid cooling of the metal from theaustenitizing temperature to a point above the Ms where it is held for aspecified period of time, followed by cooling in air. There are three types ofinterrupted quenching: austempering, marquenching (martempering), andisothermal quenching. The temperature at which the quenching isinterrupted, the length of time the steel is held at temperature, and the rateof cooling can vary depending on the type of steel and workpiece thickness.


31/39

Comparison of cooling rates and temperature gradients as workpieces pass into andthrough martensite transformation range for a conventional quenching and temperingprocess and for interrupted quenching processes. (a) Conventional quenching andtempering processes that use oil, water, or polymer quenchants.

(b) Marquenching, which uses either salt or hot oil as a quenchant. (c) Austempering,which uses a salt as a quenchant. (d) Isothermal quenching, which uses either salt or

hot oil as a quenchant.


32/39

TEMPERING OF STEEL

a process in which previously hardened ornormalized steel is usually heated to atemperature below the lower critical temperatureand cooled at a suitable rate, primarily to

increase ductility and toughness, but also toincrease the grain size of the matrix.

Steels are tempered by reheating afterhardening to obtain specific values ofmechanical properties and also to relievequenching stresses and to ensure dimensionalstability.


33/39

Principal Variables

Tempering temperature

Time at temperature

Cooling rate from the temperingtemperature

Composition of the steel, including carboncontent, alloy content, and residualelements


34/39

Structural Changes

Based on x-ray, dilatometric, and microstructuralstudies, there are three distinct stages oftempering, even though the temperature rangesoverlapStage I: The formation of transition carbides and

lowering of the carbon content of the martensite to0.25% (100 to 250 C, or 210 to 480 F)

Stage II: The transformation of retained austenite to

ferrite and cementite (200 to 300 C, or 390 to 570 F)Stage III: The replacement of transition carbides and

low-temperature martensite by cementite and ferrite(250 to 350 C, or 480 to 660 F)


35/39

Effect of tempering temperature on room-temperature mechanicalproperties of 1050 steel. Properties summarized are for one heat of1050 steel that was forged to 38 mm (1.50 in.) in diameter, thenwater quenched and tempered at various temperatures.Composition of heat: 0.52% C, 0.93% Mn


36/39

Effect of tempering temperature onthe mechanical properties of oil-quenched 4340 steel bar. Singleheatresults: ladle composition, 0.41% C,0.67% Mn, 0.023% P, 0.018% S,0.26% Si, 1.77% Ni, 0.78% Cr,0.26% Mo; grain size, ASTM 6 to 8;critical points, Ac1, 730 C (1350 F);Ac3, 770 C (1415 F); Ar3, 475 C(890 F); Ar1, 380 C (720 F);treatment, normalized at 870 C(1600 F), reheated to 800 C (1475

F),quenched in agitated oil; crosssection, 13.46 mm (0.530 in.) diam;round treated, 12.83 mm (505 in.)diam; round tested; as-quenchedhardness, 601 HB.


37/39

Notch toughness as a function of temperingtemperature for 4140 (UNS G41400) ultrahigh-strength steel tempered 1 h


38/39


39/39

Date post:	05-Apr-2018
Category:	Documents
Upload:	haerul-atamimi
View:	217 times
Download:	0 times

05 Heattreatment

Documents