Progressive low-hardenability steels

Abstract

A new class of progressive steels, called low-hardenability (LH) steels, has been developed and come into broad industrial use in Russia. These steels are intended for use in a new method of strengthening important heavily loaded machine parts -bulk-surface quenching (BSQ).The fact that a lower hardenability is obtained in making these steels compared to structural carbon steels makes it possible to use their rapid quenching by fast-moving water to obtain a high-strength (HRC 60), ultrafine-grained surface layer and a strengthened core in the cross section of parts being given the strengthening treatment. For many heavily loaded machine parts, having such a cross section makes it possible to attain record high strength properties and a more durable, more reliable part.Several automobile and tractor plants are successfully using LH steels as a replacement for alloy steels that must be subjected to length, labor-intensive treatment when used for such parts as pinions, spiders in universal joints, and heavy-duty shafts.The metallurgical industry of the USSR produced 130,000 tons of steels of this type in 1990. In the machine-building sector, the trend was toward increasingly broader application of LH steels. However, there was no accompanying expansion of the range of LH steels produced, due to the sharp decline in machine production.The Chusovoi Metallurgical Plant, supported by the Kama River Automobile Factory (KAMAZ), has implemented an important and timely program to further the production and commercial use of LH steels.Although three grades of steel of this class (58-55PP, 47GT, and ShKh4) have already been incorporated into the existing GOSTs, only recently has the full potential of these materials begun to be realized in industry.The use of LH steels for heavily loaded machine parts is efficient from technical and economic standpoints due to the following factors, which do not entail material expenditures by industry.The need to reduce the hardenability of the steel for the BSQ process and the limitingly high cooling rate used in this case make it necessary to sharply reduce the concentration of alloying elements in the steel. Table 1 shows some typical compositions of LH steels that were made in the course of introducing the given quenching operation at the Chusovoi plant.The high structural strength and durability of parts made of these steels can be attributed to two factors. First of all, a unique industrial technology for making LH steel was developed. That technology ensures an extremely low probability of austenite grain ~owth during the strengthening heat treatment. For example, the technology for making LH steel that was developed and introduced at the Chusovoi plant makes it possible to obtain a grain size corresponding to 11-12 points in heat-treated parts (the average area of the grains is 60--30 btm2). Conventional steels in the same state have a grain size corresponding to 7-8 points (average area of the grains 500--1000 ~tm 2, i.e., the grains are t0-15 times coarser). The existence of such ...