Research On A Low-temperature Stress-strain Relation For Austenitic Stainless Steels And Its Application

Cryogenic pressure vessels?CPV?,made of austenitic stainless steels?ASS?,is the major low-temperature equipment associated with storing and transferring of liquid oxygen,liquid nitrogen and liquid natural gas and so on.Due to the progresses in liquefaction technology,in the form of cryogenic liquified gases are widely adopted for industrial gases and energy gases.The CPV's quantity in demand has got a significant increase because of the increasing in cryogenic liquified gases.ASS's strength is significantly improved by cryogenic temperature and strain-induced martensitie caused by low-temperature deformation.However,the Chinese pressure vessel standards GB/T150-2011 and GB/T18442-2011 still rules that the mechanical properties at room temperatures should be chosen to design the CPVs.This design mechod does not fully use the low-temperature mechanical properties and results into a high weight and the waste of materials.It's more reasonable to design CPV based on mechanical properties at cryogenic temperatures instead of at room temperature.Designing a CPV based on the mechanical properties of the ASS at its cryogenic operating temperature fully utilizes the potential of the material at low temperatures,resulting in lightweight and compact products.Financially supported by the National Key R&D Programs of China?No.2016YFC0801905 and No.2016YFC0801500?and the Fundamental Research Funds for the Central Universities?No.2018FZA4013?,this paper chooses S30408 as the main research material and makes deep research on S30408's low-temperature mechanical properties,stress-strain relation and its application in elastic-plastic analysis design of CPVs.The main work of this paper is as follows:?1?A systematic series of tensile tests and impact tests were carried out in a wide range of 77–293 K,to investigate the low-temperature mechanical properties of S30408base metal and welded joints under different conditions?including different temperatures,room pre-strain and strain rates?.With temperature decreasing from 293K to 77 K,the yield strength(R_p0.2)and ultimate tensile strength?R_m?of S30408 have an average increase of 64.1%and 125.6%,respectively,which illustrates the obvious low-temperature strengthening effects.The decrease in the ability of preventing crack propagation during impact tests is the main reason results into worse impact toughness at cryogenic tempeartures.The Charpy absorbed energy and lateral expansion have a average decrease of 43.2%and 39.3%,respectively,at 77 K.9%room pre-strain contributes to a increase of 28.9%in R_p0.2and has little influence on R_m.when strain rate increases from 0.0003 s^-1 to 0.3 s^-1,the R_m and percentage elongation after fracture have a average reduction of 15.2%and 14.5%,respectively,which shows softening effects acused by adiabatic temperature rise and dynamic strain aging.In addition,ferrite content presents a radial pattern in the PAW+GTAW and SAW welded joints'cross-section.High ferrite content results into a reduction in low-temperature mechanical properties of welded joints.?2?Combining with the martensitic transformation during low-temperature deformation,the influence of strain-induced martensite on deformation is analyzed and a stress-strain relation for S30408 at low temperatures is proposed.Higher martensitic transformation rate at low temperatures results into higher strain hardening,which makes the stress-strain curve show a shape transformation from a parabolic shape to an inverted“S”shape.The widely-used Ludwigson model has not considered the effects of the martensitic transformation on deformation and exhibits a low-temperature limit.To precisely describe the stress-strain relation at cryogenic temperatures,a modified Ludwigson relation is proposed based on the assumption that the mechanical work difference between experimental curve and fitted Ludwigson curve is linearly proportional to martensite content.Compared with some typical room stress-strain relations and low-temperature stress-strain realtions,propsed realtion shows smaller fitting relatve errors and better fitting results.Because the relation parameters can rationally reflect the influences of factors,including temperature,pre-strain and strain rate,on low-temperature deformation,proposed relation can be used ro describe the deformation of S30408 at low temperatures.?3?Based on the common features in the low-temperature stess-strain curves of 15ASS types listed in Chinese standards GB/T24511-2017,propsed relation can be used to describe the low-temperature deformation of different ASS types.Considering the effects caused by differences in chemical composition of different ASS types on low-temperature mechanical properties and martensitic transforamtion,the incidence relations between low-temperatue mechanical properties and relation parameters are founded based on tensile tests of 351 groups of different ASS types.Thus,the method of calculating the low-temperature stress-strain curves of ASS in elastic-plastic analysis design is proposed.Furthermore,the low-temperature stress-strain curves of S30408 at77-293 K are calculated,which rationally reflects the influences of low temperature on mechanical properties.