Effect of Tempering on the Bainitic Microstructure Evolution Correlated with the Hardness in a Low-Alloy Medium-Carbon S
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BAINITIC steels are used in applications such as crankshafts, seat belt buckles, rails or bearings, all of which require high performance in terms of hardness and wear resistance combined with a good toughness. When choosing the optimal material for a certain component, geometrical constraints together with desired properties are determining factors. Alloying must be used to obtain the desired hardenability in order to avoid the formation of unwanted phases leading to poor mechanical properties or thermal cracking, but at the same time the cost should be minimized. The alloying content is therefore kept to a minimum and the processing time is kept short to reduce cost. The bulk mechanical properties of a steel product can often be achieved by a single heat treatment; however, additional processing steps such as coating in order to
ADAM STA˚HLKRANTZ is with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 114 28 Stockholm, Sweden and also with the Husqvarna Group, 561 82 Huskvarna, Sweden. Contact e-mail: [email protected] PETER HEDSTRO¨M and ANNIKA BORGENSTAM are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology. NIKLAS SARIUS, HANS-A˚KE SUNDBERG, and SO¨REN KAHL are with the Husqvarna Group. MATTIAS THUVANDER is with the Department of Physics, Chalmers University of Technology, 412 96 Go¨teborg, Sweden. Manuscript submitted May 5, 2020; accepted September 9, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
enhance wear, corrosion, and friction properties of the surface may result in further processing steps at elevated temperature resulting in tempering of the bulk steel. Bainite was first observed in 1920 by Hultgren,[1] but was at that time identified as ‘‘secondary ferrite.’’ Still, a century after the discovery, the full extent of the formation of bainite[2–4] and how the structure correlates to the mechanical properties is not completely understood. There are several studies on the contribution of different bainitic microstructural features to the mechanical properties.[5–10] Much focus has been on the effect of the bainitic ferrite sub-units and the retained austenite, but the carbon content and the dislocation density in the ferrite have also been discussed. It should be emphasized that many of the previously investigated steels contained high amounts of Si which are of interest thanks to their excellent mechanical properties, yet their main drawback is long processing times.[11] When processing time is an important factor, low Si bainitic steels are more suitable. It has been reported that bainitic ferrite in high Si grades is rather stable upon tempering and only small changes in the width of packets have been observed for tempering up to 30 minutes at 550 C.[9] The high Si content destabilizes the cementite to such an extent that it may not precipitate; instead the remaining austenite in such a case is enriched with carbon and is thus stabilized and remains as films and blocks between the plates and packets of the bainit
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