Owing to the heat treated steel strips and stainless steel sheets are so widely used the world over that the heat-treating procedures are continuously under research, there are chiefly three different treating procedures.
One is the conventional quenched and tempered process which is normally quenched indirectly by water, or directly by oil, or combined; and its product is tempered martensite.
Another is called martempering, which is quenched by lead bath or salt bath:and its product is also tempered martensite but with better quality, the third is Austempering, which is also quenched by lead bath or salt bath; and its product is bainite.
The two procedures of martempering and Austempering, quenching in lead have considerable advantages over the conventional quenched and tempered procedure, and have largely displaced the latter.
Technology of microstructure transformation
In Fig. I it shows the conventional quenching and tempering procedure.
The strip passes from the hardening temperature (approx.820-920°C, depending on the type of steel) to below the martensite point in one step. This is a typical abrupt oil quench. With this technique of cooling in one step, the strip must cope with a change in volume caused by rapid cooling down, and with crystal elongation generated by martensite forming, all at the same time. The result is a correspondingly higher level of residual stress in the strip.
In contrast to this, Fig.II represents a process of quenching in lead, with martensite forming after quenching, The molten lead-referred to as the lead quench from now on-has a temperature of approx.300°C, so quenching from austenitizing temperature of the quench is less abruqt than in the case of oil quenching, but still rapid enough to steer clear of the pearlite " cape " shown in the transformation diagram. During the first stage of cooling the austenite is retained,and the strip can adjust to the change in volume caused by cooling down, Before bainite can start to form, the strip leaves the lead quench and passes through a system of air jets, where it is gently further cooled to below the martensite point. Here the strip can adjust to the crystal elongation resulting from transformation into martensite, without undergoing any change in volume through abrupt cooling, The result is thus a lower and more uniform level of stress, compared with oil quenching; the steel strip transformed using a lead quench can therefore be treated better.
Fig.III shows the time / temperature pattern for austempering. The lead quench is at a higher temperature here, in the range 350 to 370°C. Cooling down fast to this level keeps the strip microstructure austenitic, and bainite starts forming in the lead quench.