Carbon Content In Martensite Research Articles

Effect of dual-phase treatment and tempering on the microstructure and mechanical properties of a high strength, low alloy steel

The role of morphology and martensite carbon content in the microstructure and mechanical behaviour was investigated for a 1Cr-1Mn dual-phase high strength, low alloy steel (where the composition is in approximate weight per cent), subjected to different quenching techniques. The effect of tempering for 1 h at 200–600°C on the structure and properties of the dual-phase steels thus produced was studied. The changes in microstructure were investigated by scanning and transmission electron microscopy. The yield behaviour, strength and ductility were determined from tensile tests, whereas the hardness of each phase, i.e. of martensite and ferrite, was measured individually as a function of tempering temperature. A relation between the strength and ductility was obtained and the results of tensile and microhardness measurements were discussed in the light of the changes in microstructure.

Influence of martensite carbon content on the cyclic properties of dual-phase steel

A series of FeMnC alloys was quenched from 760° and 820°C. This treatment produced dual-phase microstructures in which the carbon contents of the martensite and ferrite phases were held constant while the percent martensite varied. The monotomic and cyclic properties of these steels were determined and the major influence on mechanical properties was found to be the percentage of martensite; both monotonic and cyclic stress levels increase linearly with martensite content. Carbon content of the phases also appears to play a role, particularly for cyclic properties. At constant martensite contents higher carbon levels result in better fatigue properties. Thus dual-phase steels with a higher alloy content (therefore able to be more slowly cooled resulting in higher carbon bearing martensite) may be preferable for cyclic applications.

急速加熱焼入されたニッケル・クロム・モリブデン浸炭鋼の機械的性質

As the present author reported previously, the surface hardness and toughness of carburized Ni-Cr-Mo steel which was heated rapidly to the optimum austenitizing temperature and quenched afte holding at that temperature for a short time were higher than those of steels which was slowly heated and quenched after holding for a longer time.To invcstigate the reason of the above result, static bending test, rolling-bending fatigue test, investigation of tempering characteristics as well as the metallographic examination in electron microscope were carried out.The results are summarized as follows:(1) The optimum austenitizing conditions for JIS SNCM21 steel carburized to content of 0.73%C are the quenching temperature of 800°, the holding time of 5 minutes, and the minimum heating rate of 30° per minutes.(2) From the results of static bending test and rolling-bending fatigue test, it was found that the beneficial effect of rapid heating and short time holding treatment was very small in steels containing less than 0.50%C.(3) The rapid heating and short time holding result in lower carbon content in martensite, higher Ms temperature and finer austenitic grain size compared with the case of the slow heating and long time holding.(4) Lower carbon martensite with undissolved carbide and fine grain size are attributed to rapid heating and short time holding.From these reasons and analysis of the thermal expansion curves of the specimens of different carbon content, it is clarified that the specimens treated by rapid heating and short time holding can be tempered at lower temperatures than those treated by slow heating and short time holding.

過時効残留オーステナイトのマルテンサイト変態

The martensite transformation of over-aged retained austenite was investigated using nickel steels. The martensite transformation showed the complicated changes by aging. These changes consist of four stages (see Fig. 1). The lowering of Ms′ point in the 1st stage is caused by the stabilization of retained austenite. The rising of Ms′ point in the 2nd stage is caused by the stress relaxation due to martensite decomposition during aging. The lowering of Ms′ point in the 3rd stage is caused by the increase in the martensite-austenite interfacial free energy because of the lowering of carbon content in martensite and the concentration of these partial carbon atoms at the martenite-austensite interface during aging. The lowering of Ms′ point in 4th stage is caused by the stabilization of retained austenite due to partial transformation to martensite during cooling from the aging temperature.