Research regarding heat treatment influence on properties of chromic-manganese steel with quenching in polymer solution with purpose of matching drill-stem subs

According to GOST 4543-71, steel refers to the group of chromium-manganese steels. It is a medium-alloy constructional steel of pearlite class. The paper presents a theoretical and experimental research activity carried out in order to improve the properties of the 38HGM steel. The results permit the selection of the right heat-treatment parameters that offer the best combination among the yield strength, tensile strength, elongation, necking, impact strength at low temperature and hardness, in order to obtain a better behavior of the material for production of drill pipe subs.


Introduction
This paper describes the research activity carried out in order to improve the drill string equipment quality and performance and to extend its field of application. The problems having been analyzed and studied refer to the key-elements of that type of equipment, namely the bodies being under pressure. Extending the field of application of the drill string equipment to its use under pressure requires the use of some material brands capable to meet the requirements [2].
The 38HGM steel, one of the pearlite steels, was selected to be studied. Its properties meet the requirements in order to be used for manufacturing castings used within the drill string equipment that works under high pressure working condition. The steel smelting procedure and the influence of the steel chemical composition on the material behavior are of high importance. This paper presents theoretical and experimental research regarding the influence of the heat-treatment on the mechanical properties of the 38HGM steel.

Research methodology
The research regarding the heat-treatment influence on the mechanical properties was carried out by going deeply into the following fields of research: ♦ theoretical study of the recommendations offered by the specialty literature regarding the parameter values of the heat treatment applied to that steel; ♦ experimental study of the mechanical properties of samples with the same chemical composition; in those samples the authors applied a heat-treatment schedule, consisting in quenching at the certain temperature with cooling in polymer solution, and one temper operation;

Theoretical research
The 38HGM ferritic-pearlitic structure can be varied by the heat treatment so that a wide range of hardness (450 to 550HB) and other mechanical properties can be obtained. The structure of the preeutectoid steel after annealing consists of excess ferrite and perlite [5]. Depending on the temperature of heat-treatment, the hardened alloy exhibits a pearlitic to martensitic structure that results in a tough, erosion-resistant material. The melting temperature is 1510 C. For maximum softness, castings may be annealed at minimum 788 C and usually between 843 C and 898 C and slowly furnace cooled.
The alloy is hardened by heating to 870 C and cooling in oil. [GOST 4543] After hardening, a part blank should be tempered as soon as possible in the range of 580 -620 C. Tempering in the vicinity of the range of 250-480 C should be avoided because low impact strength will result [4]. The highest strength and hardness are obtained by tempering at 250 C or below, but this also leads to a sharp drop in the toughness (figure 1, where 1 -air cooling, 2 -water/polymer solution cooling). It is important to note that the mechanical properties shown in Table 1 are given for samples with a diameter of 25 mm and a length of about 70 mm, and the indicated thermal treatment regime does not guarantee mechanical properties in large billets with a wall thickness of about 30 mm.
According to GOST 7360-82 Drill-stem subs, it is necessary to obtain the following mechanical properties for drill-stem subs: Tensile The study was performed on the blank shown in Figure 2. The billet is made of steel 38HGM, melting № 3072, a circle of assortment 210; the chemical composition is presented in table 2.  Heat treatment was carried out with cooling in the polymer solution, named as UZSP-1. The concentration of the polymer solution is 2%. UZSP-1 (TU 38-403192-86) is a water-alkaline solution of methacrylonitrile copolymer. According to the study [2], the solution of this polymer composition with different concentrations can be used at temperatures of 20 С ... 40 С. It can be used for a long time without replacement and, most importantly, the cooling rate curve of this solution lies between the curves of water and oil. But the thermal conductivity of this system is much higher than the thermal conductivity of the oil (because the polymer is water-based), and due to this the authors obtain  The results of hardness measurements are presented in Table 4. Hardness was measured in accordance with GOST 9012-59 Metals. The method of Brinell hardness measurement (ISO 410-82) was used.

Table 4. Hardness test results
After measuring the hardness of the billet, it was checked for cracks by the non-destructive testing method. During the control, cracks in the workpiece were not detected. Further, from the billet, three control blanks were prepared for making samples for mechanical testing. The sketch of cutting is shown in Figure 3.

Conclusion
Theoretical and experimental studies indicate that it is necessary to study heat treatment technologies with quenching in a polymer solution in order to satisfy the requirements of GOST 7360 Drill pipe adapters.
Analyzing the obtained mechanical properties, the conclusions are as follows: 1. Thermal treatment in a polymer solution can significantly improve the mechanical properties of the steel. 2. Technological processing of blanks in the polymer environment ensures compliance with the requirements of GOST 7360. 3. A solution with this concentration provides protection against cracking, in contrast to quenching in water.