Thermal Processing of Metals

Chapter 11

Annealing A heat treatment process in which a material is heated to an elevated temperature, allowed to dwell there for a set amount of time and then cooled with a controlled rate.

Stages of annealing:

• Heating to required temperature

• Holding (“soaking”) at constant temperature

• Cooling

The time at the high temperature (soaking time) is long enough to allow the desired transformation (diffusion, kinetics) to occur.

Cooling is done slowly to avoid warping/cracking of due to the thermal gradients and thermo-elastic stresses within or even cracking the metal piece.

• Purposes of annealing:

• • Relieve internal stresses

• • Increase ductility, toughness, softness

• • Produce specific microstructure

Process Annealing used to revert effects of work hardening (by recovery and recrystallization) and to increase ductility. Heating is usually limited to avoid excessive grain growth and oxidation. Oxidation may be avoided by performing at relatively low temperature (but above the recrystallization temperature) or in nonoxidizing temperature.

Stress Relief Annealing used to eliminate/minimize stresses arising from

o Plastic deformation during machining

o Non-uniform cooling

Annealing temperatures are relatively low so that useful effects of cold working are not eliminated.

Annealing of Ferrous Alloy

• Lower critical temperature A1 below which austenite (γ) does not exist.

• Upper critical temperature lines, A3 and Acm above which all material is austenite (γ).

• The presence of other alloying elements will shift the eutectoid and the positions of these phase boundary lines.

Eutectoid Point

Hypoeutectoid reaction

Hypereutectoid reaction

Normalizing

• An annealing heat treatment just above the upper critical temperature to reduce the AVERAGE grain sizes (of pearlite and proeutectoid phase) and make more uniform size distributions.

• Heat to at least 55°C above A1 for euctectoid and above A3 for composition less than 0.76 wt. % C and above Acm for compositions more than 0.76 wt. %.

• After complete transformation to austenite (austenitizing - γ). the treatment is completed by cooling to the required microstructure.

Full anneal

• Austenizing (γ) and slow cooling (several hours). Produces coarse pearlite -large grains (and possible proeutectoid phase) that is relatively soft and ductile. Full annealing is used to soften pieces which have been hardened by plastic deformation, and which need to undergo subsequent machining/forming.

• The alloys is treated above 50°C above A3 or A1. Then, cooled slowly in furnace.

• Microstructure is coarse pearlite ( in addition to proeutectoid phase).

Spheroidizing

• Prolonged heating just below the eutectoid temperature, which

results in the soft spheroidite structure. This achieves maximum softness needed in subsequent forming operations. Can take place by several methods such as:

(a) Heating just below A1 (transformation time from pearlite~ 15 – 25h)

(b) Heating to a temperature just above eutectoid temperature; cooling slowly in the furnace or holding at a temp just below the eutectoid temp

(c) Heating and cooling alternately within about +50°C of A1.

Heat Treatment of Steels

Martensite has the strongest microstructure and can be made more ductile by tempering. Therefore, the optimum properties of quenched and tempered steel are realized if a high content of martensite is produced.

Problem: It is difficult to maintain the same conditions throughout the entire volume of steel during cooling: the surface cools more quickly than interior, producing a range of microstructures throughout. The martensitic content, and the hardness, will drop from a high value at the surface to a lower value in the interior of the specimen.

Production of uniform martensitic structure depends on

• composition

• quenching conditions

• size + shape of specimen

Tempering - Hardness

• Martensite is the hardest / strongest and most brittle of the steel microstructures

• Hardness is a function of carbon content

• Enhance ductility by tempering.

• Anneal to equilibrium ferrite plus cementite phases. Formation by this route called tempered martensite.

Hardenability

• Hardenability is the ability of the Fe-C alloy to be hardened by forming martensite.

• Hardenability is not “hardness”. It is a qualitative measure of the rate at which hardness decreases with distance from the surface because of decreased martensite content.

• High hardenability means the ability of the alloy to produce a high martensite content throughout the volume of specimen.

• Hardenability is measured by the Jominy end-quench test, performed for standard cylindrical specimen, standard austenitization conditions, and standard quenching conditions (jet of water at specific flow rate and temperature).

Hardenability: Jominy end-quench test of Hardenability

The “Hardenability Curve” is the dependence of

hardness on distance from the quenched end.

• After removal from furnace, specimen is quickly mounted as shown. The lower end is quenched by a jet of water cooling rate is maximum at the quenched end.

• After cooled at RT, shallow flats of 0.4 mm deep are ground along the specimen length and Rockwell hardness measurement are made.

Hardenability: Hardenability Curve

• Quenched end cools most rapidly and contains most martensite

• Cooling rate decreases with distance from quenched end

• High hardenability means that the hardness curve is relatively flat.

Less martensite

Hardenability

• Alloying elements delay formation of pearlite,

bainite : more martensite

• Can also define hardenability in terms of

cooling rate (C/s)

Hardenability curves for five different steel alloys, each containing 0.4 wt. % C. Approximate alloy compositions (wt %) are as follows:

(a) 4340 1.85 Ni, 0.80 Cr, 0.25 Mo. (b) 4140 1.0 Cr, 0.2 Mo. (c) 8640 0.55 Ni, 0.5 Cr, 0.2 Mo. (d) 5140 0.85 Cr. (e) 1040 Unalloyed steel

Hardenability also generally

increases with C content

Hardenability

Hardenability: Influence of Quenching Medium, Specimen Size, and Geometry on Hardenability

Quenching medium: Cooling is faster in water then oil, slow in air. Fast cooling brings the danger of warping and formation of cracks, since it is usually accompanied by large thermal gradients.

The shape and size of the piece: Cooling rate depends upon extraction of heat to specimen surface. Thus the greater the ration of surface area to volume, the deeper the hardening effect. Spheres cool slowest, irregularly shaped objects fastest.

Radial hardness profiles of cylindrical steel bars

Hardenability: Specimen size and rate of cooling

Cooling rate as a function of diameter at surface, three quarter radius (3/4 R), midradius (R/2), and center positions for cylindrical bars quenched in mildly agitated (a) water and (b) oil.

Dis

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rom

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Example Problem 11.1

Determine the radial hardness profile for a 50 mm (2 in) diameter cylindrical specimen of 1040 steel that has been quenched in moderately agitated water.

(a)The cooling rate at the center of a water-quenched 50 mm (2 in) diameter is determined (Figure 11.7a)

(b)The cooling rate is converted into an HRC hardness for 1040 steel. (Figure 11.14)

(c)The Rockwell hardness is plotted on the radial hardness profile (Figure 11.18).

Exercise 1

• Construct radial hardness profiles for the following:

• (a) A 45-mm diameter cylindrical specimen of an 8640 steel alloy that has been quenched in

• moderately agitated oil

• In the manner of Example Problem 11.1, the equivalent distances and hardnesses tabulated below were determined from Figures 11.14 and 11.17b

• Radial Equivalent HRC • Position Distance, mm Hardness • Surface 7 52 • 3/4 R 11 50 • Mid radius 14 45 • Center 16 44

• The resulting hardness profile is plotted below.

Exercise 1

• A cylindrical piece of steel 20 mm in diameter is to be quenched in moderately agitated oil.

• Surface and center hardnesses must be at least 55 and 50 HRC, respectively. Which of the following alloys will satisfy these requirements: 1040, 5140, 4340, 4140, and 8640? Justify your choice(s).

solution In moderately agitated oil, the equivalent distances from the quenched end for a 20-mm diameter bar for surface and center positions are 3 mm and 8 mm respectively [Figure 11.17b]. The hardnesses at these two positions for the alloys cited (as determined using Figure 11.14) are given below

Surface Center Alloy Hardness (HRC) Hardness (HRC)

1040 50 30 5140 56 49 4340 57 57 4140 57 55 8640 57 53

Thus, alloys 4340, 4140, and 8640 will satisfy the criteria for both surface and center hardnesses.

Exercise 2

• A cylindrical piece of steel 70 mm in diameter

is to be austenitized and quenched such that a

minimum hardness of 40 HRC is to be produced throughout the entire piece. Of the alloys 8660,

8640, 8630, and 8620, which will qualify if the quenching medium is (a) moderately agitated

water, and (b) moderately agitated oil? Justify your choice(s).

Solution

• This problem calls for us to decide which of 8660, 8640, 8630, and 8620 alloys may be fabricated into a cylindrical piece 70 mm in diameter which, when quenched in mildly agitated water, will produce a minimum hardness of 40 HRC throughout the entire piece.

• The center of the steel cylinder will cool the slowest and therefore will be the softest. In moderately agitated water the equivalent distance from the quenched end for a 70-mm diameter bar for the center position is about 15 mm [Figure 11.17a]. The hardnesses at this position for the alloys cited (Figure 11.15) are given below.

Center

Alloy Hardness (HRC)

8660 58

8640 42

8630 30

8620 22

Therefore, only 8660 and 8640 alloys will have a minimum of 40 HRC at the center, and therefore, throughout the entire cylinder

• (b) This part of the problem asks us to do the same thing for moderately agitated oil. In moderately agitated oil the equivalent distance from the quenched end for a 70-mm diameter bar at the center position is about 22.5 mm [Figure 11.17b]. The hardnesses at this position for the alloys cited (Figure 11.15) are given below.

Center

Alloy Hardness (HRC)

8660 53

8640 37

8630 26

8620 < 20

Therefore, only the 8660 alloy will have a minimum of 40 HRC at the center, and therefore, throughout the entire cylinder

Exercise 3

• A cylindrical piece of steel 40 mm in diameter is to be austenitized and quenched such that a

• microstructure consisting of at least 80% martensite will be produced throughout the entire piece. Of the alloys 4340, 4140, 8640, 5140, and 1040, which will qualify if the quenching medium is (a) moderately agitated oil and (b) moderately agitated water? Justify your choice(s).

Since the cooling rate is lowest at the center, we want a minimum of 80% martensite at the center position. From Figure 11.17b, the cooling rate is equal to an equivalent distance from the quenched end of 12 mm. According to Figure 11.14, the hardness corresponding to 80% martensite for these alloys is 50 HRC.

Solution

• Alloy Hardness (HRC) • 4340 56 • 4140 53 • 8640 49 • 5140 43 • 1040 25

Thus, only alloys 4340 and 4140 will qualify.

Thus, all we need do is to determine which of the alloys have a 50 HRC hardness at an equivalent distance from the quenched

end of 12 mm. At an equivalent distance of 8 mm, the following hardnesses are determined from Figure 11.14 for the various

alloys.

(b) For moderately agitated water, the cooling rate at the center of a 40-mm diameter specimen is 8 mm equivalent distance from the quenched end [Figure 11.17a]. At this position, the following hardnesses are determined from Figure 11.14 for the several alloys.

Alloy Hardness (HRC) 4340 57 4140 55 8640 54 5140 51 1040 33

It is still necessary to have a hardness of 50 HRC or greater at the center; thus, alloys 4340, 4140, 8640, and 5140 qualify

fig_11_14

Alternative quiz:

A cylindrical piece of steel 85 mm in diameter is to be quenched in moderately agitated water. Surface and center hardnesses must be at least 55 and 40 HRC, respectively. Which of the following alloys will satisfy these requirements: 1040, 5140, 4340, 8640 fand 4140? Justify your choices

fig_11_15

fig_11_16

fig_11_17

fig_11_18

fig_11_19

fig_11_20

Precipitation Hardening

• Small inclusions of secondary phases strengthen material

• Lattice distortions around these secondary phases impede dislocation motion

• The precipitates form when the solubility limit is exceeded

• Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time.

Heat Treatment for Precipitation Hardening

3) Precipitation heat treatment: the supersaturated solution is heated to T2 where diffusion is appreciable – β phase starts to form as finely dispersed particles: aging.

1) Solution heat treatment: at To, all the solute atoms A are dissolved to form a single-phase (α) solution.

2) Rapid cooling across the solvus line to exceed the solubility limit. This leads to a metastable supersaturated solid solution at T1. Equilibrium structure is α+β, but limited diffusion does not allow β to form.

Schematic temperature vs time plot showing both

solution and precipitation heat treatments for

precipitation hardening.

Heat Treatment for Precipitation Hardening

With increasing time, the strength or hardness increases, reaches a maximum, and finally diminishes. This reduction on strength and hardness that occurs after a long time period is known as overaging.

The precipitation hardening characteristics of a 2014 aluminum alloy (0.9 wt. % Si, 4.4 wt. % Cu, 0.8 wt. % Mn,

0.5 wt. % Mg) at four different aging temperatures (a) yield strength, and (b)

ductilility (% EL).

The strengthening process is accelerated as the temperature is increased.

Associated with an increase in strength is

a reduction in ductility.

Is it possible to produce a precipitation hardened 2014 aluminum alloy having a minimum yield strength of 350 MPa (50,000 psi) and a ductility of at least 18% EL? If so, specify the precipitation heat treatment. If it is not possible, then explain why.