Austempering versus Quenching and Tempering: Impact Behavior of SAE 4140 and SAE O1 Steels Lauralice de C. F. Canale, José E. Nucci University of São Paulo, São Carlos, SP, Brazil
[email protected] [email protected] Jan Vatavuk Universidade Presbiteriana Mackenzie, São Paulo, SP, Brazil
[email protected] George E. Totten Portland State University, Portland, OR, USA
[email protected]
Introduction It is well known that austempering, producing a microstructure bainitic, is the preferred heat treatment when toughness is the main requirement of the component. Literature shows a several works where mechanical properties for tempered martensite are compared with bainite presenting both same hardness. Most part of the results demonstrate better properties for the bainite microstructure. There are 2 types of bainite, upper bainite formed from austenite transformation in a isothermal bath using higher range of temperature, usually 500⁰C. Lower bainite is formed at lower temperatures, about 350⁰C, depending on the alloy steel. Lower bainite has hardness very close to the tempered martensite and it is used when elastic characterisitics are wished. This work intends to verify impact properties in 2 different alloy steels, SAE 4140 (Cr-Mo) and SAE O1, (high carbon tool steel) submitted to different heat treatment cycles. Tempered martensite obtained from the conventional heat treatment followed or not cryogenic bath are compared with bainitic structure produced in austempering. Since hardness has influence on the impact behavior, comparative studies were made for the same surface hardness level. This means that hardness for bainite and tempered martensite in the case of SAE 4140 are similar each other. Bainite and tempered martensite for SAE O1 presents same hadness level. These steels were chosen because they are very common to make components and tools for the industry. Since they have different C amounts in their composition, it will be possible to compare if this parameter (%C) will bring change in the results.
Experimental Procedure Table 1 shows parameters used for the conventional quenching process for both steels. Table 1: Parameters used for quenching process Alloy Steel
Austenitization Temperature (⁰C)
Soaching Time (min)
Quenching media
SAE 4140
860
60
Oil at 60⁰C
SAE O1
820
60
Oil at 60⁰C
Tempering temperature was chosen after a study where the embrittlement temperature range was determined for both steels. Embrittlement temperature to SAE 4140 is about 370⁰C and to SAE O1 this temperature is close to 300⁰C. Have in mind this temperatures and also, in order to achieve (with the tempering) same hardness as that obtained after austempering, tempering temperature for SAE 4140 was 400⁰C during 1 hour. In the case of SAE O, tempering temperature was 450⁰C for 1 hour. Some samples were additionally submitted to the cryogenic bath during 1 hour. Cryogenic bath is used mainly for high C alloy in order to transform retained austenite. Austempering process is considered a isothermal heat treatment and for both steels same isothermal temperature was chosen, according Table 2. Table 2: Parameters used for the austempering process Alloy Steel
Austenitization Temperature (⁰C)
Soaching Time (min)
Austempering Temperature (⁰C)
Austempering Process Time (min)
SAE 4140
860
60
350
40
SAE O1
820
60
350
40
Hardness values (HRC) were obtained and Impact Charpy tests were performed according ASTM E23-1.
Results and Discussion Table 3 summarizes the obtained results for all the heat treatment tested conditions. Clearly is possible to observe higher values of absorbed energy for tempered martensite, independent on the C amount of the alloy. Cryogenic bath brings a small increase in this behavior, also for both steels. Such results put in discussion the assertion about the dominance of the bainitic structure on the tempered martensite related to impact behavior. Although there are some works in the literature which corroborate results here found, most part of them conclude the inverse [1]. Confirming tempered martensite as more toughness than bainite, Santos, Goldenstein and Vatavuk [2] show results from experiment involving studies about impact toughness behavior in modified martempering followed by tempering, austempering and conventional quenching and tempering. They concluded that microstructure from martempering and tempering gave better performance than those obtained from other heat treatments. Under those studied conditions, bainite and tempered martensite obtained from conventional quenching and tempering has similar behaviors in terms of toughness. Zepter [3], Meng-Yin et al[4] afirm that for the same hardness level, lower bainite has more toughness and ductility than tempered martensite. Question is verify if in those works, tempering of the martensite was performed out of the embritllement range of temperature. It is well know that during the tempering the toughness can be decreased because of the “tempered martensite embrittlement (TME)”. For sure in such cases, the toughness performance will be impared and bainite will show better behavior.
Table 3: Hardness results and Absorved Impact Energy results from the different heat treatment processes.
Alloy Steel
SAE 4140
Energia de Impacto médio (kJ/m²)
Deviation (kJ/m²)
46.62
1.34
4.94
0.10
45.28
1.55
13.50
0.50
45.11
2.46
16.69
1.10
39.13
1.82
12.98
3.12
40.97
1.12
33.00
1.00
39.80
1.85
35.23
1.54
Hardness (HRC)
Austempering Quenching & Tempering Quench&Temper & Cryo Austempering
SAE O1
Deviation (HRC)
Heat Treatment
Quenching & Tempering Quench&Temper & Cryo
Conclusions Considering the heat treatment parameters and test conditions performed in this work it can be concluded that for same hardness level quenching and tempering followed by cryogenic bath gave better performance in terms of impact toughness for both studied steels. Even comparing with tempered martensite without cryogenic bath, bainite microstructure did not show higher values of absorved energy.
Key References [1] BARREIRO, J. A. Tratamientos térmicos de los aceros, Plaza de Sta. Ana, Madrid, 8ª. ed., editorial dossat, 1985. [2] SANTOS, D. F.; GOLDENSTEIN, H., VATAVUK, J. The modified martempering and its effect on the impact toughness of a cold work tool steel, São Paulo, Congresso SAE BRASIL 2011. [3] ZEPTER, P. G. Influencia da microestrutura nas propriedades mecânicas do aço ABNT 5160 em condições de carregamento monotômico e cíclico. 2007, Dissertação mestrado, IPEN - USP, São Paulo (In Portuguese). [4] MENG-YIN TU; CHENG-AN HSU; WEN-HSIUNG WANG; YUNG-FU HSU. Comparison of microstructure and mechanical behavior of lower bainite and tempered martensite in JIS SK5 steel. Materials Chemistry and Physics 107. 2008. p. 418–425.
