The invention refers to hardening heat treatment of the perlite-class steel.
A process of (TSH) heat treatment—is known where the selection or development of the steel chemical composition was frequently based on the necessity requirement to ensure both through- and surface thermal hardening of the part by alloying so as to increase hardenability; or the ability of the steel to harden in weak cooling media to prevent crack formation. Alloying is also less frequently used to improve the steel quality features compared to carbon steels—i.e., to increase strength, plastic and dynamic properties, heat resistance, lower the cold brittleness threshold, etc.
A process of surface hardening with deep induction heating is known, now called through-surface hardening.
It was also known that many mechanical properties can be achieved using the low (LH) and specified hardenability (SH) carbon- and low-alloy steels.
These are steels whose hardenability complies with the effective loaded cross-section of the part; in this case, following TSH, the surface layers of this cross-section, i.e., 0.1-0.2 of the diameters (thickness), have a martensite structure with HRC=60 hardness, while the core hardness is HRC=30-45.
Steel hardenability corresponds to the ideal diameter (DI) value of the steel that actually defines the optimum hardened layer depth with reference to the specific part shaped as a cylinder, sphere or plate.
In principle, LH and SH steels are used for the same purpose and just conventionally differ only by the ideal diameter (DI) value: for LH steels as in the earlier products, this diameter is equal to 8-16 mm, whereas for modern SH steels it is over 16 mm.
The necessary DI range of LH and SHI steels for a specific type of parts was achieved in an improvement by a combined restriction in the upper limit of one or a group of added elements which led to a lower accuracy and a wider DI range.
A disadvantage of the LH steels (see patent RU 2158329) is that the specified low hardenability value was achieved only by a strict limitation of all permanent elements—Mn, Si, Cr, Ni, Cu, which made smelting more difficult and resulted in lower DI range accuracy during the development of the chemical composition and, as a consequence, led to a wider deviation in the hardened layer depth and exceeded tolerance ranges.
For example, LH steel with 0.8% C and containing Mn, Si, Cr, Ni, Cu (<1% each and 0.06-0.12% Ti (LH steels)—(see patent RU 2158320) has minimal hardenability—DI<12 mm for austenite size 10 grain and finer grain (#11) DI<11 mm and <10 mm for #12 grain size, whereas similar steel with 0.8% C; 0.05% Mn; 0.12% Si; 0.11% Cr, 0.25% Ni; 0.3% Cu; 0.05% Al; 0.22% Ti (with a wider range of Ni, Cu) has the same DI value.
The object of this invention is to provide a process for heat treatment during induction and furnace heating using LH and SH steels which avoid the disadvantage described.
Another object is to obtain an even finer austenite grain ##11-13 GOST5639 (ASTM), more stable preset hardenability level (DI), with a substantially smaller deviation and which strictly corresponds to the depth of the hardened layer obtained on parts subjected to heat treatment using the proposed procedure an improved process, the capability of treating thinner, smaller parts with the through-surface and through-thickness hardening.