PSI - Issue 40

A.V. Vydrin et al. / Procedia Structural Integrity 40 (2022) 450–454

451

2

А.V. Vydrin at al./ Structural Integrity Procedia 00 (2022) 000 – 000

1. Introduction Nowadays, the applicable scope of tubular products made of special steels is extended. Tubular products are used for new oil and gas fields development complicated by the presence of carbon dioxide and hydrogen sulfide. Further, pipes of corrosion-resistant and stainless steels are widely used in nuclear industry, aircraft engineering, at liquefied natural gas manufacturing plants and for other knowledge-intensive production branches. To ensure high-strength and corrosion-resistant properties of steel, a sufficiently large percentage of alloying elements, such as chromium, nickel, molybdenum and etc. are added into steel. However, these elements usually impair steel plastic properties in the process of pipe manufacturing by pressure treatment. Consequently, cracks and breaks may occur on hot-worked pipes. Therefore, the process of pipe manufacture engineering requires reliable information concerning ductility of steels used for pipe production. 2. Materials and methods It is well-known Vydrin (2020) that ductility of metals and alloys is a function which depends, besides chemical composition and structure, upon the stress state and temperature scheme. These functions were named ductility curves Kolmogorov (2001). The procedure of metal and alloy curves plotting during cold-working is sufficiently described in research papers of native and foreign scientists. In this case ductility only depends upon the stress state scheme defined by the average normal stress σ to shear stress intensity T ratio. Stress state diagram can be changed by carrying out experiments with tension and torsion with application of liquid external hydrostatic pressure Bogatov (1984). Numerous research works carried out using this technique have demonstrated that the dependence of ductility on the stress condition diagram is well described by exponential dependence. However, use of this technique for construction of metal ductility diagram under hot deformation is fraught with significant difficulties. This is primarily due to the fact that production of hot-formed pipes occurs at temperatures of 950 ... 1250 ° C and experiments with application of liquid hydrostatic pressure up to 10000 atmospheres have to be carried out using special disposable devices and fluids for heating Smirnov (1994), Bogatov (1995), Smirnov (1997), Lapovok (2000). It should be noted that research works concerning ductility of metals and alloys in hot conditions are carried out, including recently, for example Nesterenko (2005). However, the mentioned research works are not yet systemic, and there are no generally accepted approaches, such as for cold deformation. On the other hand, scientific and technical progress in the sphere of testing unit development led to creation and widespread occurrence of universal test complexes such as Gleeble Rudskoy (2010). Availability of such facilities opens up new possibilities for studying of metal and alloy ductility. Taking into account the opportunities that have opened up, it is proposed to use tensile, torsion and upsetting trials using the Gleeble 3800 test complex for ductility determination. In this case, it is assumed as working hypothesis that the effect of the stress condition diagram on ductility is described by exponential dependence at any test temperatures. Since such dependence implies determination of two empirical coefficients, experiments with two types of deformation are sufficient for its construction. For example, these can be tensile and upsetting trials. Nevertheless, while carrying out additional type of torsion tests, it is possible to evaluate applicability of the working hypothesis concerning independence of nature of influence of stress condition diagram on ductility on the test temperature. When choosing the type of dependence describing the ductility diagram of metals and alloys during hot deformation, its linearization possibility was taken into account to apply the method of least squares when processing the experimental results. As a result, the following formula is proposed:



a a 





 

  

  

 

1 12



exp

a   

a

0

2

p

1000

 

(1)