SHORT BAINITE TREATMENT OF STEELS FOR BETTER PROPERTIES
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Expert enquiry: We help you in case of problems SHORTENED BAINITIC TREATMENTS OF STEEL SHOWING INCREASED PROPERTIES News category : Column By recent research projects of IWT Bremen it was demonstrated experimentally that either specified combined bainitic– martensitic, or two-step bainitic treatment cycles (Tab. 1, Fig. 1: B1M* and BB), lead to a significant shortening of the heat treatment time, whilst maintaining hardening for steels and avoiding distortions. As the results show, the duration can be reduced to about 20% of the full bainitic treatment of cold- working and roller- bearing steels. Fig.1 Isothermal TTT Diagram of the steel 100Cr6 (SAE 52100) showing the thermal cycles and pre-quenching (qpre) Cycles Practical process data Austenitisation Heat treatment Mqt 845°C 20 min/ (salt bath) quenched in oil to RT / 170°C 2h B 230 °C 4 h B1M* 230 °C 25 min / H2O RT + 250 °C 5 min Bincomplete 230 °C 50 min / H2O RT 845BB 230 °C 50 min + 250 °C 5 min / H2O RT Tab. 1 Heat treatment notation according to DIN EN 10052 The amount of transformed bainite can be varied by control of the duration of the isothermal treatment in the lower bainitic stage according to dilatometric curves. Up to 70 vol.% of transformed bainite, a microstructure composed of bainite, martensite and retained austenite was obtained after post-quenching and tempering. As found by experiments, the tempering-temperature should be chosen in the same range as the bainitisation temperature, or slightly raised and the tempering duration reduced. By means of this procedure the heat treatment can be shortened to about 15 – 25 % of the time for complete bainitic transformation. Two-step bainitic treatment also leads to the same reduction of the heat treatment duration in comparison with complete bainitisation. With regard to the transformation-time-temperature (TTT-) diagram (Fig. 1) the second step of the consecutive heating should be raised to temperature ranges above the previous temperature stage and below the average temperature where bainitisation starts first. Required hardness levels can be adjusted by the temperature range chosen for the second treatment, either using the bainitic-martensitic-cycle (B1M*) or the two-step bainitisation process (BB). The austenitisation temperature should be selected at the lowest possible point above Ac1e (Fig. 1) to obtain a fine-grained primary structure, and the temperature for bainitisation should be set narrowly above the martensite start temperature Ms. As shown by two-step bainitisation, the volume fraction of less saturated austenite will be decomposed to carbide and ferrite by subsequent short tempering at an enhanced temperature range (Fig. 1 BB). Nearly similar results can be achieved after the combined bainitic– martensitic treatment and subsequent short tempering at elevated stages above Ms, as shown with the cycle B1M* (Fig. 1). It should be noted, that the retained austenite is then trapped between the bainite platelets at a sub-microscopic scale (Fig. 2), as mentioned above. Because of the fine distribution of the retained austenite, no deterioration of the mechanical properties or hardness occurs. The choice of low austenitisation temperature demands a homogeneous primary structure for the steel, which can be achieved either by a previous heat treatment process or by premium casting procedures, spray-forming, or powder production. Fig. 2 Steel 100Cr6: Lower Bainite with retained austenite (RA) after incomplete transformation [Field Emission SEM (15 keV)] of an electrolytically thinned TEM foil] The hardness measured at room temperature (Fig. 3) decreases with decreasing martensite- and increasing bainite-content in the Print this page HEATREAT.DE - The Heat Treatment Consultant Network -- -- News: Sh... http://www.heatreat.de/en/news/detail/149/Column/Shortened-bainitic-t... 1 of 2 23/12/2014 4:22 PM
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HEAT TREATMENT OF STEELS TO OBTAIN BETTER PHYSICAL AND THERMAL PROPERTIES , SHOCK LOADING , INCREASED FATIGUE LIFE AND WEAR RESISTANCE
Transcript of SHORT BAINITE TREATMENT OF STEELS FOR BETTER PROPERTIES
Expert enquiry:We help you in caseof problems
SHORTENED BAINITIC TREATMENTS OF STEEL SHOWING INCREASED PROPERTIES
News category : Column
By recent research projects of IWT Bremen it was demonstrated experimentally that either specified combined bainitic–martensitic, or two-step bainitic treatment cycles (Tab. 1, Fig. 1: B1M* and BB), lead to a significant shortening of theheat treatment time, whilst maintaining hardening for steels and avoiding distortions.
As the results show, the duration can be reduced to about 20% of the full bainitic treatment of cold- working and roller-bearing steels.
Fig.1 Isothermal TTT Diagram of the steel 100Cr6 (SAE 52100) showing the thermal cycles and pre-quenching (qpre)
Cycles Practical process data
Austenitisation Heat treatment
Mqt
845°C 20 min/(salt bath)
quenched in oil to RT / 170°C 2h
B 230 °C 4 h
B1M* 230 °C 25 min / H2O RT + 250 °C 5 min
Bincomplete 230 °C 50 min / H2O RT
845BB 230 °C 50 min + 250 °C 5 min / H2O RT
Tab. 1 Heat treatment notation according to DIN EN 10052
The amount of transformed bainite can be varied by control of the duration of the isothermal treatment in the lower bainitic stage according todilatometric curves. Up to 70 vol.% of transformed bainite, a microstructure composed of bainite, martensite and retained austenite was obtainedafter post-quenching and tempering. As found by experiments, the tempering-temperature should be chosen in the same range as thebainitisation temperature, or slightly raised and the tempering duration reduced. By means of this procedure the heat treatment can be shortenedto about 15 – 25 % of the time for complete bainitic transformation.
Two-step bainitic treatment also leads to the same reduction of the heat treatment duration in comparison with complete bainitisation. With regardto the transformation-time-temperature (TTT-) diagram (Fig. 1) the second step of the consecutive heating should be raised to temperatureranges above the previous temperature stage and below the average temperature where bainitisation starts first. Required hardness levels canbe adjusted by the temperature range chosen for the second treatment, either using the bainitic-martensitic-cycle (B1M*) or the two-stepbainitisation process (BB).
The austenitisation temperature should be selected at the lowest possible point above Ac1e (Fig. 1) to obtain a fine-grained primary structure,and the temperature for bainitisation should be set narrowly above the martensite start temperature Ms. As shown by two-step bainitisation, thevolume fraction of less saturated austenite will be decomposed to carbide and ferrite by subsequent short tempering at an enhanced temperaturerange (Fig. 1 BB). Nearly similar results can be achieved after the combined bainitic– martensitic treatment and subsequent short tempering atelevated stages above Ms, as shown with the cycle B1M* (Fig. 1).
It should be noted, that the retained austenite is then trapped between the bainite platelets at a sub-microscopic scale (Fig. 2), as mentionedabove. Because of the fine distribution of the retained austenite, no deterioration of the mechanical properties or hardness occurs. The choice oflow austenitisation temperature demands a homogeneous primary structure for the steel, which can be achieved either by a previous heattreatment process or by premium casting procedures, spray-forming, or powder production.
Fig. 2 Steel 100Cr6: Lower Bainite with retained austenite (RA) after incomplete transformation[Field Emission SEM (15 keV)] of an electrolytically thinned TEM foil]
The hardness measured at room temperature (Fig. 3) decreases with decreasing martensite- and increasing bainite-content in the
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microstructure. The two-step bainitisation with austenitisation at 845 °C (Table 1: 845BB) has not reduced the hardness. The shortened heattreatment cycle B1M* (Table 1), achieving a microstructure composed of bainite, martensite and about 8 vol.% of retained austenite showsequivalent hardness values in comparison to complete bainitisation (B).
Fig. 3
Rotating bar fatigue tests show of the curves the fracture probability versus stress amplitude, and the values extrapolated at PB = 50 % within thediagram (Fig. 4), revealed that the fatigue strength was increased by the shortened treatments of bainitic transformation followed by martensiticpost-quenching and tempering (B1M*), incomplete bainitisation (Bincomplete) and two-step bainitisation (BB1). With these test conditions thefatigue strength values after complete bainitisation (B) and martensitic quenching and tempering (Mqt) show inferior results compared to thecombined heat treatment cycles.
Fig. 4
Vetters, H., J. Dong, H. Bomas, F. Hoffmann and H.-W. Zoch: Microstructure and fatigue strength of the roller-bearing steel 100Cr6 (SAE52100) after two-step bainitisation and combined bainitic- martensitic heat treatment. Int. Journal of Mat. Res.: Vol. 97 (2006), p. 1432-1440
Author: Prof. Dr. Vetters
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