THE ASSESSMENT OF IMPACT OF CARBONITRIDING ON THE LOCATION OF COLD-BRITTLENESS THRESHOLD IN THE MEDIUM-CARBON ALLOY STEELS

Last 10–15 years, in the world industrial practice, a great interest for the processes of low-temperature saturation of iron alloys with the nitrogen or with the nitrogen and carbon is observed. The galloping technology – carbonitriding – is one of such methods. The key point of the process is in the simultaneous saturation of steel surface with the nitrogen and carbon in the molten salts at a temperature of 540...580 °C. In the result of carbonitriding, high hardness is achieved on the product surface, and the wear-resistance, fatigue strength and stain-resistance increase.

However, despite the significant achievements in this direction, many issues are still not clarified or remain a subject of discussions, including the influence of heating temperature for carbonitriding on the possibility of the reversible temper brittleness manifestation in the medium-carbon alloy steels. In this paper, the impact of carbonitriding on the properties of base metal, in particular, its tendency to brittle fracture was studied. The authors determined the change of location of cold-brittleness threshold in the 30HGSA, 40HN2MA, 38HMA, and 40G2 steels both in the initial condition and after carbonitriding. Using the method of fractographic analysis, the authors determined the nature of fracture of steel study specimens and evaluated the shear area fraction of impact specimens. It is shown, that the tendency to temper embrittlement in the study steels is different and depends significantly on their chemical composition. The paper presents the results of field tests of drill pipes with screw joints made of 30HGSA, 40HN2MA, 38HMA, and 40G2 steels in various climatic zones. The authors recommend steels to produce tool joints for drill pipes using the carbonitriding methods for the thread face-hardening for the operation at low temperatures.