Heat Treatment of Steels

Transcription

1 MME444 Heat Treatment Sessional Week Heat Treatment of Steels Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Common Heat Treatment of Steels 1. Annealing 3. Hardening 2. Normalising 4. Tempering Purposes of annealing Refining grains Inducing ductility, toughness, softness Improving electrical and magnetic properties Improving machinability Relieve residual stresses Purposes of normalising Modifying and refining cast dendritic structure Refining grains and homogenising the structure Inducing toughness Improving machinability Purposes of hardening Improving hardness Improving wear resistance Purpose of tempering Relieving residual stresses Improving ductility and toughness (at the sacrifice of some hardness or strength) 1

2 Example 12.1 Design of a Method to Determine AISI Number An unalloyed steel tool used for machining aluminum automobile wheels has been found to work well, but the purchase records have been lost and you do not know the steel s composition. The microstructure of the steel is tempered martensite, and assume that you cannot estimate the composition of the steel from the structure. Design a treatment that may help determine the steel s carbon content. Example 12.1 SOLUTION The first way is to heat the steel to a temperature just below the A 1 temperature and hold for a long time. The steel overtempers and large Fe 3 C spheres form in a ferrite matrix. We then estimate the amount of ferrite and cementite and calculate the carbon content using the lever law. If we measure 16% Fe 3 C using this method, the carbon content is: ( x ) % Fe3C or x ( ) 1.086% A better approach, however, is to heat the steel above the A cm to produce all austenite. If the steel then cools slowly (annealing), it transforms to pearlite and a primary microconstituent. If, when we do this, we estimate that the structure contains 95% pearlite and 5% primary Fe 3 C, then: x % Pearlite or x % 2

3 Example 12.2 Determination of Heat Treating Temperatures Recommend temperatures for the process annealing, annealing, normalizing, and spheroidizing of 1020, 1077, and steels. Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels. Example 12.2 SOLUTION From Figure 12.2, we find the critical A 1, A 3, or A cm, temperatures for each steel. We can then specify the heat treatment based on these temperatures. 3

4 Example 11.8 Design of a Heat Treatment to Generate Pearlite Microstructure Design a heat treatment to produce the pearlite structure shown in Figure Figure Growth and structure of pearlite: photomicrograph of the pearlite lamellae ( 2000). (From ASM Handbook, Vol. 7, (1972), ASM International, Materials Park, OH ) Example 11.8 SOLUTION Interlamellar spacing of the pearlite: Figure The effect of the austenite transformation temperature on the interlamellar spacing (in cm) of pearlite. If we assume that the pearlite is formed by an isothermal transformation, we find from Figure that the transformation temperature must have been about 675 o C. 4

5 Figure The time-temperaturetransformation (TTT) diagram for an eutectoid steel. From the TTT diagram (Figure 11.21), our heat treatment must have been: 1. Heat the steel to about 750 o C and hold perhaps for 1 h to produce all austenite. A higher temperature may cause excessive growth of austenite grains. 2. Quench to 675 o C and hold for at least 10 3 s (the P f time). 3. Cool to room temperature. Example 11.9 Heat Treatment to Generate Bainite Microstructure Excellent combinations of hardness, strength, and toughness are obtained from bainite. One heat treatment facility austenitized an eutectoid steel at 750 o C, quenched and held the steel at 250 o C for 15 min, and finally permitted the steel to cool to room temperature. Was the required bainitic structure produced? 5

6 Example 11.9 SOLUTION A banitic structure can only be obtained during isothermal cooling of austenite, commonly known as austempering. Figure The time-temperature-transformation (TTT) diagram for an eutectoid steel. After heating at 750 o C, the microstructure is 100%. After quenching to 250 o C, unstable austenite remains for slightly more than 100 s, when fine bainite begins to grow. After 15 min, or 900 s, about 50% fine bainite has formed and the remainder of the steel still contains unstable austenite. Thus, the heat treatment was not successful!! The heat treatment facility should have held the steel at 250 o C for at least 10 4 s, or about 3 h. Example 12.3 Design of a Heat Treatment for an Axle A heat treatment is needed to produce a uniform microstructure and hardness of HRC 23 in a 1050 steel axle. Figure 12.2 (a) The Fe-Fe 3 C phase diagram. Figure 12.8 The TTT diagrams for a 1050 steel. 6

7 Example 12.3 SOLUTION 1. Austenitize the steel at (30 to 55) = 805 o C to 825 o C, holding for 1 h and obtaining 100%. 2. Quench the steel to 600 o C and hold for a minimum of 10 s. Primary ferrite begins to precipitate from the unstable austenite after about 1.0 s. After 1.5 s, pearlite begins to grow, and the austenite is completely transformed to ferrite and pearlite after about 10 s. After this treatment, the microconstituents present are: Primary α (0.77 ( ) ) % Pearlite (0.5 ( ) ) % 3. Cool in air-to-room temperature, preserving the equilibrium amounts of primary ferrite and pearlite. The microstructure and hardness are uniform because of the isothermal anneal. Example 12.4 Design of a Quench and Temper Treatment A rotating shaft that delivers power from an electric motor is made from a 1050 steel. Its yield strength should be at least 145,000 psi, yet it should also have at least 15% elongation in order to provide toughness. Design a heat treatment to produce this part. Figure 12.8 The TTT diagrams for a 1050 steel. Figure The effect of tempering temperature on the mechanical properties of a 1050 steel. 7

8 Example 12.4 SOLUTION 1. Austenitize above the A 3 temperature of 770 o C for 1 h. An appropriate temperature may be = 825 o C. 2. Quench rapidly to room temperature. Since the M f is about 250 o C, martensite will form. 3. Temper by heating the steel to 440 o C. Normally, 1 h will be sufficient if the steel is not too thick. 4. Cool to room temperature. Week 2-4: Heat Treatment of Steels Part 1: Design of a heat treatment cycle of a steel sample 1. Analyse your steel sample to determine the carbon and other alloys, if any, contents. 2. Determine the approximate cooling rate and quenching medium required to obtain the desired properties. Finally select the heating temperature and holding time required and plot the heat treatment cycle of the process. Part 2: Conduct the heat treatment cycle 1. Once the heat treatment cycle is approved by the course tutor, conduct the heat treatment operation. 2. After heat treatment, prepare a metallographic sample from your heat treated steel sample and obtain micrographs in different magnifications. 3. Measure hardness of your heat treated sample in Rockwell C scale. 8

9 Work Schedule Student Group Sample Description Desired Properties 1 AISI AISI AISI AISI AISI The steel to be quenched and tempered to produce a minimum yield strength of 1000 MPa and a minimum of hardness VHN 40 The steel to be quenched and tempered to produce a structure having a tensile strength of at least 1050 MPa but a hardness below RC 40 Apply a suitable heat treatment to produce a structure containing pearlite and martensite Apply a suitable heat treatment to produce a fully martensitic structure and then temper enough to have a hardness within the range of RC Apply a suitable heat treatment to make the steel soft enough to be machined and have a hardness below RC 45 Supplementary Tables and Figures Ref: D. A. Askeland, The Science and Engineering of Materials, 4th Ed., Chapman & Hall,

10 Figure 12.2 (a) The Fe-Fe 3 C phase diagram. Figure 12.5 The effect of carbon and heat treatment on the properties of plain-carbon steels. 10

11 Figure 12.4 Schematic summary of the simple heat treatments for (a) hypoeutectoid steels and (b) hypereutectoid steels. Figure The effect of interlamellar spacing (λ) of on the yield strength of pearlite. 11

12 Figure The effect of the austenite transformation temperature on the interlamellar spacing of pearlite. Figure Increasing carbon reduces the M s and M f temperatures in plain-carbon steels. 12

13 Figure The time-temperature-transformation (TTT) diagram for an eutectoid steel. Figure 12.8 The TTT diagrams for a 1050 steel. 13

14 Figure 12.8 The TTT diagrams for a steel. Figure The CCT diagram (solid lines) for a 1080 steel compared with the TTT diagram (dashed lines). 14

15 Figure The CCT diagram for a low-alloy, 0.2% C Steel. Figure The effect of tempering temperature on the mechanical properties of a 1050 steel. 15

16 Figure Effect of tempering temperature on the properties of and eutectoid steel. Figure Formation of quench cracks caused by residual stresses produced during quenching. The figure illustrates the development of stresses as the austenite transforms to martensite during cooling. 16

17 Figure The marquenching heat treatment designed to reduce residual stresses ands quench cracking. 17