Effect of Tempering Temperature on Microstructure

International Journal of Manufacturing, Materials, and Mechanical Engineering, 4(3), 33-49, July-September 2014 33

Effect of Tempering Temperature on Microstructure, Texture and Mechanical Properties of a High Strength Steel Pradipta Kumar Jena, Defence Metallurgical Research Laboratory, Hyderabad, India K. Siva Kumar, Defence Metallurgical Research Laboratory, Hyderabad, India A.K. Singh, Defence Metallurgical Research Laboratory, Hyderabad, India

ABSTRACT This work describes the microstructure, texture and anisotropy in mechanical behavior of a high strength steel in various tempered conditions. The microstructures and mechanical properties change considerably with varying tempering temperatures. The material exhibits low in-plane anisotropy and low anisotropic index in terms of yield strength and elongation with increase in tempering temperature. The anisotropy of the material displays similar behavior to that of the yield strength. Keywords: Ferrous Metals And Alloys, Heat Treatments, Microstructure, Texture, X-Ray Diffraction

1. INTRODUCTION For long, steels have been used as armour due to their high strength combined with good toughness and low cost. Generally, quenching and tempering

are well-established means to achieve strengthening in steel. Considerable knowledge exists on the effect of alloy compositions and the heat treatments on mechanical properties (Barbacki et al., 1998; Lee et al., 1999; Srivastava et

DOI: 10.4018/ijmmme.2014070102 Copyright © 2014, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited.

34 International Journal of Manufacturing, Materials, and Mechanical Engineering, 4(3), 33-49, July-September 2014

al., 2006; Ray et al., 2003; Dhua et al., 2001). Heat treatments of steels exhibit a range of mechanical properties. As a result, the same material can be used in many applications. Tempering of steel at lower temperatures achieves higher strength and hardness, and it is used in armour applications. On the other hand, higher tempering temperatures achieve lesser hard steels with increased toughness levels, which find application in structural parts in armoured vehicles. The mechanical behavior of quenched-and-tempered steel depends strongly on its microstructure. There are several well-known phases in steel such as ferrite, pearlite, bainite, martensite and austenite. Each of them has different mechanical properties (Carlson et al., 1979; Chai et al., 1987, Callister, 1994). For example, martensite phase exhibits the highest level of strength in steel however, it displays limited ductility due to stress associated with it (Briant et al., 1979; Kwon et al., 1988; Horn et al., 1978). Tempering of martensite introduces phase changes in steel. Tempering at lower temperatures results in decomposition of martensitic phase to low carbon martensite and ε carbide. With increase in tempering temperature, martensite further decomposes to ferrite and cementite. It is well known that the crystallographic texture can significantly affect the mechanical properties of the rolled materials (Hsun et al., 1980). As a matter of fact, specific textures have been deliberately produced in steel sheets and utilized advantageously in commercial applications. However, the effect of

texture on the properties of hot rolled, quenched and tempered plates has been less explored. In an earlier investigation on 5 Ni armour steel, Hu et al. (1980) have demonstrated that the ballistic performance of the strongly textured plates is substantially superior to that of the random textured plates of the same hardness. A combination of (112) + (111) texture has displayed higher ballistic performance in comparison to (110) and (111) type textures. It has been observed that the intensity of the texture also influences other mechanical properties like through thickness tensile and compressive strength, impact energy, fracture toughness and fatigue life. The present work thus describes the effect of tempering temperature on the microstructure, texture and mechanical properties of a high strength steel.

2. EXPERIMENTAL The chemical composition of the steel is given in Table 1. The steel was industrially hot rolled to a thickness of 25 mm. Samples were again hot rolled at a temperature of 1100°C to 5 mm from the initial thickness in a laboratory rolling mill. The specimen was deformed 10% in each pass and rolling direction was kept strictly unidirectional. After each pass, the sample was kept back into the furnace to re-attain the rolling temperature. Samples of 150 x 150 mm cross section were cut and subjected to heattreatment for modifying the microstructures and mechanical properties. For

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Table 1. Analytical chemical composition of the experimental alloy Material

Chemical Composition (wt.%)

Steel

0.3−0.35 C, 0.2−0.3 Si, 0.5−0.7 Mn, 1.4−1.7 Cr, 1.5−2.0 Ni, 0.3−0.5 Mo, 0.1−0.2 V, 0.02 Al, balance Fe

getting different quenched and tempered martensite phase, samples from the hot rolled plates were austenitised at 910°C and held at this temperature for 12 minutes followed by quenching in oil to get a fully martensitic phase. The austenitisation temperature has been taken from the previous heat treatment studies on this steel (Jena et al., 2010). The plates were then immediately tempered at temperatures of 200, 300, 400, 500, 600°C for 24 minutes followed by cooling to room temperature in air. Austenitizing and tempering were carried out in a neutral atmosphere furnace. Microstructural characterization of specimens was done in three sample planes namely, normal direction (ND), transverse direction (TD), and rolling direction (RD). The specimens were prepared following standard metallographic techniques used for steel. All the samples were etched using 2% Nital (2 ml Nitric acid and 98 ml Methyl Alcohol) to reveal the microstructure. Microstructures were examined using optical and scanning electron microscopes (OM and SEM). The X-ray diffraction (XRD) studies of bulk samples were carried out using a Philips 3020 diffractometer with CuKα radiation equipped with a graphite monochrometer. The texture was measured on sheet specimens of the tempered materials of

25 mm x 15 mm size. The ‘inel G3000’ texture goniometer coupled with curved position sensitive detector with CuKα radiation was employed for texture measurement using Schultz back reflection technique (Schultz, 1949). These pole figures were corrected for defocusing and absorption. The tensile properties of the tempered specimens were evaluated at a strain rate of 4.8x10 -1s -1 using an Instron Universal Testing machine (Instron 5500R) along three test directions namely, the longitudinal (L or 0°), 45° (specimen axis at 45° to the rolling direction) and transverse (T or 90°). The schematic sample dimension is shown in Figure 1. Five specimens were tested in each condition and average values of 0.2% yield strength (σYS), ultimate tensile strength (σUTS) and total elongation (% El) were calculated from the engineering stress – strain diagrams and reported in Table 2. The yield locus plots of the specimens were determined by the Knoop hardness method used by Lee et al. (1996) based on the wheeler and Ireland approach (Wheeler et al., 1966). A MATSUZAWA (MMT-X7) Knoop hardness tester with 200g load was used for the determination of yield locus. Fifteen indentations were taken in each of the six orientations in order to have a good statistical average.



3. RESULTS AND DISCUSSION The 3D-optical microstructure of the as quenched sample is shown in Figure2. It exhibits a complete tempered martensitic structure in all the planes. The SEM micrographs of the steel in different tempered conditions are displayed in Figure 3. The microstructures consist of typical tempered martensitic structure. It can clearly be seen that increase in tempering temperature affects the microstructure of the steel (Figure 3 (a)–(e)). The coarsening of the martensitic laths with increase in tempering temperature is visible from the micrographs. The XRD patterns exhibit the presence of martensitic – Fe phase only in as quenched and tempered conditions (Figure 4). No trace of austenite phase has been observed in the tempered specimens. This is well in agreement with the microstructural observations. The lattice parameters of as quenched specimen are ‘a’ = 2.8822Å and ‘c’ = 2.8716Å. The variations of lattice parameters (‘a’ and ‘c’) as function of tempering

temperature are shown in Figure 5. It displays that the ‘a’ and ‘c’ parameters do not change with the increase in tempering temperature. It is also observed that the c/a ratio is almost equal to unity at all the tempering temperatures. This is in well agreement with other studies where it has been illustrated that the c/a ratio of the quenched Fe-C steels is equal to unity up to 0.6 wt.% carbon (Fink et al., 1926; Sherby et al., 2007). The (110) pole figures of a few selected specimens are shown in Figure 6. The pole figures are asymmetric in nature indicating the presence of triclinic sample symmetry (Bunge, 1982). The pole figures exhibit same kind of texture for all these plates, although the degree of texture intensity varies with tempering temperature. The plates tempered at 300°C display maximum intensity which is approximately 3.1 times random. The intensity increases from as quenched to 300°C tempering and then decreases with increase in tempering temperature.

Figure 1. A schematic diagram of tensile specimens showing the sample dimension (all are in mm)



Table 2. Mechanical properties of the alloy in three specimen directions Tempering Temperature (°C)

YS (MPa) 0

45

UTS (MPa) 90

0

45

% Elongation 90

0

45

90

200

1406.5

1390.5

1382.0

1858.0

1839.0

1815.0

18.8

16.5

16.2

300

1465.0

1448.0

1437.0

1711.5

1678.5

1662.5

17.0

16.0

14.4

400

1420.0

1401.5

1399.0

1615.5

1602.5

1592.0

17.45

16.9

15.2

500

1259.0

1248.0

1242.0

1393.5

1381.0

1378.5

20.4

19.2

18.0

600

1137.5

1130.0

1126.5

1264.5

1262.0

1252.0

23.6

22.2

21.3

Figure 2. 3-D Optical microstructure of the quenched steel specimen

The mechanical behavior of the steel is quite sensitive to the tempering temperature and rolling direction. The change in strength of the material

with tempering temperature is shown in Figure 7 and is presented in Table 2. The material has the highest σ UTS at 200°C tempering (Figure 7 and Table 2).



This can be attributed to low tempering temperature which does not introduce significant change in the microstructure of the martensite phase. It is known that the martensite transformation is accom-

panied by a large amount of distortion which rapidly increases the strength and hardness of steel. With increase in tempering temperature martensite lath size increases. This leads to a gradual

Figure 3. SEM micrographs of the specimens tempered at (a) 200 °C (b) 300 °C (c) 400 °C (d) 500 °C and (e) 600 °C



decrease in the σUTS values. However, the yield strength values show an increase up to 300°C tempering followed by a gradual decrease beyond that. Martensite transformation is associated with internal stresses. Tempering process relieves the internal stresses across the lath boundaries by permitting local rearrangement of atoms. Below 300°C tempering temperatures, internal stresses generated are not fully released. With complete recovery of stresses at 300°C tempering, a rearrangement of dislocation structure takes place, which restricts their movement and in turn results in an increase in the yield strength. Similar increase in σYS of high strength steels

at this stage of tempering has also been reported in literature (Jena et al., 2010; Jena et al.,2007; Malakondiah et al. 1997; Malakondiah et al. 1994; Zakharov, 1998). On tempering above 300°C, decrease in σ YS occurs due to coarsening of martensite laths and easy movement of dislocations by thermal assistance. The strength values of the tempered specimens along three directions namely, the longitudinal (0°), 45° and transverse (90°) with respect to the rolling direction are shown in Table 2. The material displays highest strength values along the longitudinal direction (0°) in all the tempered conditions followed by the 45° and transverse (90°)

Figure 4. The XRD patterns of all the tempered specimens



Figure 5. Variations of a, c and a/c with tempering temperature

Figure 6. The (110) pole figures of the specimens tempered at (a) 200 °C (b) 300 °C (c) 600 °C



Figure 7. Effect of tempering temperature on the YS and UTS of differently heat treated plates

directions, respectively. This indicates the presence of anisotropy in the material. The differences in strength values between the longitudinal and transverse directions display an increase up to 300 °C tempering. This difference narrows down significantly above 300 ºC tempering temperatures pointing towards the reduction in extent of the anisotropy in the material. Variation of hardness as function of tempering temperature is shown in Figure 8, which reflects similar trend to that of the UTS. The mechanical property anisotropy in quenched and tempered plates can be calculated by % in-plane anisotropy (AIP) and anisotropy index parameter (δ)

(Jata et al., 1996; Wu et al., 1997). The A IP is defined as: 



2σ − σYS (T ) − σYS (45)  A IP =  YS (L )  × 100 

2σYS (L )



(1)

where, σYS(L) = Yield strength in longitudinal direction. σ YS(T) = Yield strength in transverse direction. σ YS(45) = Yield strength in 45° to the rolling direction.



Figure 8. Effect of tempering temperature on hardness of different heat treated plates

It is clear from Equation (1) that the AIP is zero for isotropic materials and its value increases with increase in the extent of anisotropy in the material. The A IP of the differently tempered steel plates have been calculated and given in Table 3 and is plotted in Figure 9. It can be seen that A IP exhibits an increase upto

300°C followed by a gradual decrease beyond that. This observation is in agreement with the nature of yield strength with tempering temperature (Figure 7). The anisotropy index (δ) is defined as: δ = (ε l ~ εt) / (εl + εt)

(2)

Table 3. Anisotropy of the alloy Tempering Temperature (°C)

A IP

δ

200

1.44

0.074

300

1.54

0.082

400

1.39

0.068

500

1.11

0.062

600

0.81

0.051



Figure 9. Variations of AIP and δ with tempering temperature

Where, δ = The anisotropy index parameter εl = Elongation in longitudinal direction εt = Elongation in transverse direction The value of δ is zero for isotropic materials where εl = εt . The maximum value of the anisotropic index is 1 which corresponds to either εl >> ε t or ε t >> ε l. The anisotropic index calculated for differently tempered specimens are given in Table 3 and is plotted in Figure 9. The anisotropic index also demonstrates the agreement with AIP (Table 3). The

anisotropic index increases upto 300 °C tempering and weakens with further increase in tempering temperature. It is important to mention here that the AIP and δ are associated with anisotropy in yield strength and elongation, respectively. The values of both the parameters increases up to 300 °C tempering temperature and then decreases with increase in tempering temperature. The variation of AIP and δ as function of tempering temperature can be attributed to the presence of texture in the material. It is to be noted that the overall intensity of texture also increases up to



300 º C tempering and then decreases with increase in tempering temperature. The yield locus plots obtained by Knoop hardness method (Lee et al.,

1973) of the quenched and tempered steel plates are shown in Figure 10. The values of the tensile and compressive counter parts of Knoop hardness yield

Figure 10. Yield locus plots of the specimens tempered at (a) 200 °C (b) 300 °C (c) 400 °C (d) 500 °C and (e) 600 °C



strengths are summarized in Table 4. It is apparent that the yield strengths determined by the Knoop hardness (Table 4) appears to be 4 to 5 times that of the values determined by tensile tests (Table 2). It is important to mention here that the exact quantitative determination of the yield strength by the Knoop hardness is not possible. These values can only be used for the qualitative determination of presence of anisotropy in the material. If the yield locus plot is perfect ellipse the material is isotropic, otherwise it is anisotropic. The increase in the yield locus plot size reflects texture hardening while the values root mean square error (RMSE) exhibits the extent of anisotropy. It is worth mentioning here that the yield locus plots of thin rolled samples cannot be generated with conventional compression and tensile tests. The difference in yield loci of the steel as a function of different tempering temperatures reveals the variation of anisotropy with tempering. The differences in the tensile and compressive strengths are maximum in 300 °C temper-

ing condition indicating the presence of high anisotropy. The root mean square distance (0.01x107) between experimental points and yield locus also illustrates highest anisotropy at 300 °C tempered specimen (Table 4). This is in agreement with the measurements of the AIP and the δ values (Table 3). From Table 4, it can also be seen that the area of the yield locus plot is maximum for 300 °C tempered specimen. This matches well with the obtained yield strength values. It shows that the present material shows texture hardening upto 300 ºC tempering and then texture softening occurs during tempering the steel above 300 ºC. The texture hardening and softening as observed above can be seen as a combined effect of relieve of internal stresses and coarsening of martensite laths during tempering process.

4. CONCLUSION A correlation among microstructure, texture and mechanical properties anisotropy has been established for a

Table 4. The values of tensile and compression counterparts of the Knoop hardness yield strength values Tempering Temperature (°C)

σx (MPa) Tensile

Compression

σy (MPa) Tensile

Compression

Area of Yield Locus MPa2 x (10 7)

RMSE x(10 7)

200

4971

4419

4912

5498

8.88

.0024

300

5860

5277

5700

5907

11.41

.0069

400

4965

4565

4772

5545

8.82

.0018

500

4636

4524

4624

4757

7.65

.0017

600

4497

4423

4347

4692

7.10

.0012



high strength steel tempered at different temperatures. The steel exhibits variation in texture with change in tempering temperature. The change in anisotropy with tempering temperature is in agreement with the variation of yield strength.

ACKNOWLEDGMENT The authors wish to acknowledge Defence Research & Development Organization (DRDO) for financial support in carrying out the work and Director, DMRL for kind encouragement.



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