All-Russian Scientific and Research Institute of Tube Industry - RosNITI (Chelyabinsk, Russia):

Kosmatskiy Ya. I., Cand. Eng., Deputy Head of Department of Seamless Tubes, Chief of External Relations Activity, e-mail: kosmatski@rosniti.ru

The paper presents the results of metallographic investigations that followed as a result of manufacture of pilot batch of a new kind of difficult products with a deep degree of processing of raw material in the conditions of Volzhskiy Pipe Plant (VTZ). These products are presented by hot-extruded pipes with helical inner ribbing surface (IRS — Inner Ribbing Surface). The article notes the substantiation for carrying out the metallographic investigation and shows the laboratory equipment used for its implementation. In particular, the article presents the research of macro- and microstructures of the samples taken from hot-extruded pipes with helical inner ribbing surface, subjected to heat treatment under laboratory conditions of The Russian Research Institute of the Tube & Pipe Industries (ROSNITI JSC) to obtain a uniform ferritepearlite structure. Special attention in the article is paid to the study of the infl uence of rotational motion of the press mandrel and appearance of helical metal flow, accompanied by shear deformation, on working through the structure of hot-extruded pipes made of steel grade 20. At the same time, the paper presents the comparative assessment of microstructures of the samples of hot-extruded pipes with helical inner ribbing surface and microstructures of the samples of smooth pipes subjected to heat treatment under the relevant conditions, provided the uniform ferrite-pearlite structure. Also the paper presents the measuring results of hardness for the samples of hot-extruded pipes with helical inner ribbing surface both in longitudinal direction and cross section, including comparison of the results of hardness measurement in areas with internal ribbing. The effect of any additional helical metal flow during hot extrusion of pipes on formation of the certain structural heterogeneity (extending to the depth of ~ 3.5 mm and depending on deformation characteristics of the material used in hot extruded pipes with helical inner ribbing of surface) has been discussed.

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