Process for strengthening nodular cast iron and finished product thereof

Abstract

The invention discloses a process for strengthening nodular cast ion and a finished product thereof. The process comprises the following steps of: putting cleaned nodular cast iron parts into a carburizing furnace; performing carbonitriding, namely adding methanol and ammonia at the temperature of between 800 and 840 DEG C, wherein the addition of the methanol is 80-120 drops / minute, the adding volume of the ammonia is 1 to 5 percent of the volume of the methanol cracked gas, the carbon potential is maintained to between 0.8 and 1.2 percent, and the processing time is 2 to 4 hours; and performing quenching to obtain a part finished product. In the carbonitriding process, the decarbonization preventing technological measure is adopted, the metallographic structure of the finished product comprises a surface layer, a sublayer and a substrate in turn, the surface layer is a granular or continuously distributed compound layer, the sublayer is hidden needle-like nitrogenous martensite, retained austenite and graphite, and the substrate is thin needle-like martensite, retained austenite and graphite. The process can improve the scratch resistance, wear resistance and contact fatigue strength, can effectively prevent full decarbonization during carbonitriding of the nodular cast ion, and has the advantages of low process difficulty and low production cost.

Description

technical field [0001] The invention relates to a heat treatment process of nodular cast iron, in particular to a nodular cast iron strengthening process and its finished product, which is specifically applied to making parts such as a nodular cast iron rocker arm and a tappet of an engine. Background technique [0002] The surface strengthening process of ductile iron parts generally only includes the formation of local chill layer, surface quenching, nitriding, nitrocarburizing, surface rolling strengthening and the combination of these processes, such as chill layer quenching, ductile iron crankshaft nitrocarburizing Back fillet rolling strengthening, etc., does not include the carbonitriding process, because the application of the carbonitriding process to ductile iron parts is often considered to be superfluous - the traditional understanding is that nodular cast iron parts are not carburized or carbonitrided Co-infiltration also has sufficient hardenability, and simple...

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Problems solved by technology

[0003] In addition, since the carbon content and silicon content of ductile iron are much higher than those of low carbon steel and low carbon alloy carburized steel, the tendency of decarburization of ductile iron under heating conditions is great, and the higher the heating temperature, the easier it is to decarburize , and once the nodular cast iron is decarburized, it is easy to form a decarburized layer of full ferrite structure on the surface of the nodular cast iron part. At this time, if the full ferrite structure is to be eliminated, it must be heated and carburized at a temperature above 880°C , and then re-carbonitriding at 800-840 ° C, which not only greatly increases the difficulty and cost of the process, but also makes the hardened layer and matrix coarsen, reducing the performance of the parts

[0004] The Chinese patent publication number is CN101058887A, and the invention patent application published on October 24, 2007 discloses a heat treatment process for the gear sleeve of an automobile gearbox synchronizer. The process includes: carbonitriding, press quenching, cleaning, and tempering , detection, when the equipment fails, in the carbonitriding step, the carburizing temperature is 780-850°C, the carbon potential is 0.5-1.0%, and the nitrogen addition amount is 5-12m 3 / h, the amount of methanol added is 4-7L / h, the amount of propane added is 0.5-0.9m 3 / h, the amount of ammonia added is 0-30L / h, although the patented process uses a lower carburizing temperature and the corresponding carbon potential and carbonitriding atmosphere medium to control the carbonitriding speed, so as to ensure that the equipment Long-term heat preservation during the failure period can still prevent the oxidation, decarburization or excessive penetration of the gear sleeve parts, but the structure formed by this patented process is high-carbon martensite and retained austenite, medium-carbon martensite, Low-carbon martensite, and because the patented process has a low carbon potential when the carburizing temperature is low, that is, when the carburizing temperature is 800°C, the carbon potential is 0.55±0.05%; when the carburizing temperature is 785°C, the carbon potential is Potential is 0.55±0.05%, so it is impossible to form a carbide white layer on the surface of ductile iron parts, that is, a compound layer, and cannot improve the abrasion resistance, wear resistance and contact fatigue strength of ductile iron parts

In addition, the patented process did not take into account the strong decarburization tendency of ductile iron parts during the heating process, and did not take corresponding preventive technological measures, which is very unfavorable for the carbonitriding process of ductile iron parts, especially when carbonitriding When the carburizing furnace adopted for co-carburizing is a periodic carburizing furnace such as a pre-vacuum gas carburizing furnace, an ordinary well-type or box-type gas carburizing furnace, the adoption of anti-decarburization process measures is particularly important.

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