The quality acceptance after carburizing, quenching and tempering is based on the technical requirements of large forgings. The complete requirements for carburizing and quenching of large forgings include: surface hardness, core hardness, effective hardened layer depth, surface carbide grade, surface and heart. Metallographic structure, grain size, and surface carbon concentration and gradient concentration gradient. The latter two items are rarely used for product inspection, and they are placed in the above specific indicators for acceptance. The latter two are only used when adjusting the carburizing process.
The effective hardened layer depth (ie the depth of the carburized layer) and the grain size are checked on the sample after carburizing, the surface carbide grade, the surface and the metallographic structure of the core and the hardness are checked on the quenching furnace sample. The surface hardness is checked on the specified surface of the forging after quenching and tempering.
In addition to some of the above quantitative performance indicators, there are some requirements that can only be checked in the execution of the process record, such as surface residual compressive stress can only check the quenching final cooling temperature; carburizing depth and concentration uniformity can only be checked Furnace temperature uniformity.
There are two standards and three inspection methods for the inspection of the depth of the carburized layer. The two criteria are: a carbon concentration of the carburized layer to a depth of 0.45% C. This is the old standard. The other is to measure the hardness to the depth of the end of HV550 as the depth of the layer, called the effective hardened layer depth, which is currently the international standard.
Depth measurement method: The first is to use the magic 10mm sample to cut and observe and measure the cross-section layer after carburizing and quenching (different grains, different temper colors, see with a magnifying glass). The second is to cut the sample after annealing, polishing, and etching to see 100 times of the metallographic phase. The 1/2 of the ferrite precipitation zone (transition zone) is measured as the depth of the layer. Equivalent to a depth of 0.45% C. Of course, the most accurate carburization depth is the chemical analysis of the sample stripping.
The effective hardened layer depth is the hardness measured from the surface to the core by a Vickers hardness meter. The method and standard of measurement can be determined according to International GB9450-88 "Determination and Checking of Effective Hardening Layer of Steel Carburizing and Quenching".
The effective hardened layer depth as the depth quality standard of the carburized layer is more realistic for the design and application angle, but it brings certain difficulties to the carburizing process and operation control of the large forging, because the carburizing process can only control the carburized layer. It is not possible to directly control the hardened layer. The effective hardened layer depends not only on the depth of the carburized layer, but also on the hardenability of the part material, the quenching medium (cooling rate) and the cross-sectional size of the part (or sample). For example, the same carburized layer depth (i.e., a depth of 0.45% C measured) is 2 mm, two different materials, one is 20Cr, and the other is 20CrNi2Mo, which differs by 30% from the effective hardened layer depth of the 30 mm sample. Because for 20CrNiMO steel, the carbon content only needs to exceed 0.35% C to oil quench to HV550, while the 20Cr steel requires carbon content above 0.45% C.
In the actual carburizing process of large forgings, it is impossible to directly control the depth of the effective hardened layer. It can only be indirectly controlled by the depth of the carburized layer. As long as the depth of the effective hardened layer is related to the depth of the carburized layer under various conditions, it can be found. The effective hardened layer depth is obtained by controlling the depth of the carburized layer.