ASTM - Intensive Quenching Systems - Engineering and Design 2010 - N I Kobasko, M A Aronov, J A Powell, G E Totten

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CHAPTER 1 n THERMAL AND METALLURGICAL BASICS OF DESIGN OF HIGH-STRENGTH STEELS 9stages results in an insignificant reduction in strengthening.It has been established that the initial stages of recrystallizationexhibit a positive influence upon not only plasticity butalso fatigue strength. As for recrystallization, there is anaccompanying reduction in the yield limit [15].1.4.1 Influence High-TemperatureThermomechanical Treatment Parameterson Mechanical Properties of SteelsIn laboratory experiments and industrial tests of HTMT, differentmethods of deformation have been used: rolling of rods,strips, or bands; forging; closed and open stamping; extrusion;pressing with a pressing lathe; dragging; torsion; high-speeddeformation (by explosion). The methods that proved to bethe most effective are rolling, stamping, and forging. ForHTMT, the steel grades used in the countries of the formerUSSR are: 40 (AISI 1040), 45 (AISI 1045), U9 (AISI 1090),40Kh (AISI 5140H), 40KhN (AISI 3140), 30KhGSA (AISI5130), 50KhG (AISI 5150), 60S2 (AISI 9260), 55KhGR (AISI5155H), 65G (AISI 1566), ShKh15 (AISI 52100), 9Kh (AISI L2),and P18 (AISI T1), among others.Reduction of the deformation temperature (and itsapproach to the A C3 temperature) always exhibits a favorableeffect on the increase in strength due to HTMT. However, itis necessary to consider the slower recrystallization processesas the austenitizing temperature increases. Therefore, itis advisable to austenitize at high temperatures and todeform at temperatures close to the A C3 temperature.The effect of HTMT on the fracture strength S k andplasticity for steel 30KhGSA (AISI 5130) in the case of tensionat –320°F (–195°C) is shown in Fig. 15.The most significant improvement of mechanical propertiesis achieved in the case of HTMT with a relatively lowdegree of deformation (25–40 %). Further increase in thedegree of deformation does not yield an additional increase instrength, and changes in plastic characteristics reach the stageFig. 15—Effect of high-temperature thermomechanical treatmentupon the break strength S k and plasticity for steel 30KhGSA (AISI5130) in the case of tension at –320°F (–195°C): 1, conventionalheat treatment; 2, thermomechanical treatment [15].Fig. 16—Strength (while the plasticity is constant) and plasticity(while the strength is constant) versus degree of deformation forsteel 55KhGR (AISI 5155) in the case of high-temperature thermomechanicaltreatment [15].of saturation even at 40–50 %. The latter values of deformationdegreearethelimitsadvisableforHTMT(seeFig.16).The effect of carbon on strength in the case of HTMTis analogous to the effect of conventional heat treatment:strength increases with carbon content. Although plasticity isknown to decrease with increasing carbon content, withHTMT this tendency occurs to a lesser extent. Brittle fractureoccurs sooner than expected if the carbon content is high. Theoptimal carbon content for steel subjected to conventionalheat treatment is about 0.4 %; for steel subjected to HTMT, itis about 0.5 %. In the case of vacuum steel melting and if steelis made of especially pure charge materials, this limit value isshifted higher since the plastic strength increases.The advantage of HTMT is that higher values of plasticityare obtained up to a maximum carbon content of about0.6 %. In this case, the shear strength is high, and it is possibleto obtain a high strength of martensite when the carboncontent is high (see Fig. 17) [15].The effect of carbon appears in the fixation of dislocationstructure of strengthened austenite and also in changes in dislocationstructure of austenite during plastic deformation forthermomechanical treatment (increase in the dislocation densityand changes in the character of dislocation distribution).The addition of carbon exhibits a drastic effect on the increasein dislocation density of steel subjected to HTMT.1.4.2 Machine-Construction SteelsAt United States Steel Laboratories, Grange and his colleagues[34] investigated the application of HTMT for steelspossessing the following compositions:A. 0.26 % carbon, 0.52 % nickel, 1.32 % chromium, 1.0 %molybdenum, 0.35 % vanadiumB. 0.57 % carbon, 1.16 % nickel, 1.07 % chromium, 0.26 %molybdenumC. 0.87 % carbon, 4.95 % nickel, 2.07 % manganese
10 INTENSIVE QUENCHING SYSTEMS: ENGINEERING AND DESIGNsignificantly increases, especially in low-temperature tests, inthe area of threshold of brittleness.There is essential improvement of properties of steel4340 after HTMT at 1,550°F (845°C), with 75 % pressing out,even in the quenched state (without tempering): while theyield limit after conventional heat treatment is 1,370 MPa, afterHTMT it goes up to 1,770 MPa. This is probably connected withthe increase in the plastic strength due to the high-temperaturethermomechanical treatment. The authors of [15,36,37] havemade the conclusion that HTMT can be successfully applied forthe production of strips, sheets, and wire.Fig. 17—R m and Z versus carbon content: 1, high temperaturethermomechanical treatment; 2, conventional quenching [15].D. 0.41 % carbon, 1.90 % nickel, 0.82 % chromium, 0.28 %manganeseE. 0.25 % carbon, 0.59 % nickel, 0.73 % chromium, 0.27 %vanadiumIt has been shown by investigations that mechanical andplastic properties of machine construction steels increaseafter high-temperature thermomechanical treatment [34].Kula and Lapata [35] studied 4340 steel (0.39 % carbon,1.75 % nickel, 0.8 % chromium, 0.23 % molybdenum) duringopen melting and showed that HTMT resulted in significantstrengthening (increasing the yield limit by 25 %). In thiscase, plasticity and viscosity in the high-strength state arevery high [34,35] and the temperature where cool fractureoccurs decreases, and there is a decreased potential for fractureduring tempering (see Fig. 18). The correlation amongvalues of strength, plasticity, and viscosity on longitudinaland transverse samples after HTMT is the same as afterconventional heat treatment; impact strength after HTMTFig. 18—The scheme of low-temperature thermomechanicaltreatment.1.5 LOW-TEMPERATURE THERMOMECHANICALTREATMENT (LTMT)Low-temperature thermomechanical treatment (LTMT) isshown schematically in Fig. 18. To perform low-temperaturethermomechanical heat treatment, it is necessary to supercoolaustenite to 400–500°C, where it should be deformed toa certain degree that has the designation k df . Transformationat this time should be delayed, and supercooled austeniteshould be stable. For this purpose, as a rule, high-alloy steelsare strengthened by this method. This is an expensive processthat is rather complicated to perform. It will be shown herethat intensive quenching can replace both low-temperatureand high-temperature thermomechanical treatment.The process of LTMT requires a delay of the transformationof austenite into martensite during quenching. This isespecially important when applying intensive quenching. Duringintensive quenching, the surface temperature decreasesrapidly to the bath temperature, while temperature at the coreremains very high. Immediately at the surface, a brittle martensiticlayer is formed, which precludes mechanical deformation(see Fig. 19(a)).To successfully design a low-temperature thermomechanicaltreatment, it is necessary to delay transformation ofaustenite into martensite during the intensive quenchingprocess (see Fig. 19(b)). This may be accomplished by utilizingthe self-regulated thermal process (to be described indetail in subsequent chapters of this text) [38,39]. It isknown that a delay of the transformation of austenite intomartensite can be fulfilled by using appropriate pressure orwater salt solutions [1,38].Until now, LTMT has been used for high-alloy steels,where supercooled austenite remains when cooling at a lowrate up to 400–500°C. Unfortunately, this cannot be performedfor low-alloy and plain carbon steels. That is whyLTMT is not widely used in practice.1.6 THE USE OF THERMOMECHANICALHEAT TREATMENTThe steel grade 30KhN2MA (AISI 4330), with a chemicalcomposition of 0.27–0.34 % carbon, 0.3–0.6 % manganese,0.17–0.37 % silicon, 1.25–1.65 % nickel, 0.6–0.9 % chromium,0.2–0.3 % molybdenum, 0.05 % titanium, and less than 0.3 %copper is widely used in the industry. The mechanical propertiesof steel 30KhN2MA (AISI 4330) after conventionalheat treatment and thermomechanical treatment with thedeformation in the area of stable and metastable austeniteare presented in Table 1. Table 1 shows that ultimatestrength R m, after k df ¼ 70, increases by 11 % compared withthe conventional heat treatment. Similar results are providedin Table 2 for 40KhNM steel. Note that in all cases, after thethermomechanical treatment or conventional quenching, the
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ASTM - Intensive Quenching Systems - Engineering and Design 2010 - N I Kobasko, M A Aronov, J A Powell, G E Totten

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