THE EFFECT OF FINAL SURFACE TREATMENT OF 34CrMo4 STEEL ON RESISTANCE TO SULPHIDE STRESS CRACKING (SSC)

Transcription

1 THE EFFECT OF FINAL SURFACE TREATMENT OF 3CrMo STEEL ON RESISTANCE TO SULPHIDE STRESS CRACKING (SSC) Pavel KUČERA a, Anastasia MASLOVA a, Eva MAZANCOVÁ a a VŠB Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Czech Republic, Tř. 17. listopadu 15/2172, Ostrava Poruba, Czech Republic, pavel.kucera.st12@vsb.cz, anastasia.maslova@simd.cz, eva.mazancova@vsb.cz Abstract The 3CrMo steel is widely used material type for various applications, including high strength ones such as piping, offshore applications or materials for storage of natural gas. High level of mechanical properties is most commonly achieved by variety of heat treatment. In this paper resistance of the 3CrMo steel to sulphide stress cracking (SSC) is investigated as it is required by the certification authorities in case of materials for high pressure steel cylinders and vessels used for CNG (compressed natural gas) storage. The specimens were prepared by special machining methods to obtain three sets with different final surfaces roughness. Differences in the SSC are discussed. Keywords: High pressure steel cylinders, surface treatment, sulphide stress cracking 1. INTRODUCTION High pressure cylinders and vessels are more and more widely used in the CNG transportation and storage. In this case, great emphasis is placed on the safety of those devices. Natural gas can contain H 2 S as a contaminating element, which steel highly degrades. The basic principle of the SSC procedure is a generation of hydrogen ions by reaction between the wet hydrogen sulphide and the exposed steel. Steel microstructure can absorb this atomic hydrogen, which can be caught in hydrogen traps [1-3]. Under specific conditions in located areas hydrogen is able to increase the total localised stress exceeding the one of maternal matrix and crack formation is typical result. The SSC is influenced by inclusions and precipitates presence in matrix. Their number, size and distribution play a great role, as well as microstructure type, dislocation density, grain size, segregations, applied loading, strength level of steel and last, but not least roughness of surface including micro-notches on the surface []. Fig.1 SSC specimen a) without the notch presence, b) with the notch presence

2 Presented paper is aimed at the comparison of the influenced of variety surface machining processes applied on a high strength version of the steel 3CrMo (yield stress higher than MPa) to achieve the best SSC resistance. Presented paper is also partially targeted on overall resistance verification of such high-strength versions of mentioned steel type to the SSC. In main focus are surface roughness parameters as surface roughness R a and especially the presence of above mentioned micro-notches R t. Those micro-notches are concentrators of stresses and represents prime hydrogen traps and consequently high potential locations for preferential crack formation, see Fig. 1. On the basis of reached results one type of surface machining process as the best from point of view of the best hydrogen resistance is suggested. 2. EXPERIMENTAL PROCEDURES AND RESULTS For investigation 3CrMo steely type was used. Chemical composition of investigated steel was analysed by the optical emission spectrometer Spectrolab 2000 and is summarized in Table 1. Tab. 1 Chemical composition of 3CrMo steel (wt. %) C Si Mn Cr Mo S P N V The production of the high pressure steel cylinder (HPSC) was based on the reversed extrusion, hot rolling to the final 1 mm wall thickness and final necking (closing the spherical top of the cylinder). Specimens were cut out (extracted) of the HPSC wall after the heat treatment consisting of quenching and tempering. First two sets (further marked as HT1 and HT2) of specimens were heated at the austenitizing temperature with consequent quenching and next high temperature tempering. Last two sets of specimens were prepared by the heating in the inter-critical temperature area and quenching at the temperature of C (further marked as HT3 and HT). Preparations of specimens were based on the cutting of rough prisms, rough machining to achieve the approximate shape and cutting the threads and as a final step 3 different types of surface machining were applied. Those methods were grinding by the diamond pastes with the final maximal roughness Ra lower than μm, honing with the final roughness Ra lower than 0.1 μm and as the third method was the lapping with the Ra lower than 0.05 μm of the final roughness applied. The preparation of three types of final surface machining resulted in three sets of specimens. The evaluation of mechanical properties was also carried out. This process was based on the yield strength (YS) and tensile strength (TS), elongation (El.), notch impact energy (CVN) and hardness (HBW) testing. Mentioned procedures were necessary to confirm that the material was complying the mainly desired mechanical properties (YS higher than 1000 MPa and El. higher than 1 %). Entire testing of tensile properties was carried out using the Zwick/Roell Z 250 machine according to EN ISO standard. The testing of notch impact energy was tested by use of RKP 50 Charpy Impact Testing Machine According to ISO 18-1 standard and hardness measuring was realized on the hardness testing machine MU750 according to EN ISO standard. After the mechanical properties testing, the metallographic observation followed. Metallographic samples were prepared in longitudinal direction by the classic methods of grinding, polishing and afterward etched in Nital. Mixed microstructure contained tempered martensite, partially low bainite, acicular ferrite, eventually idiomorphic ferrite as Fig. 2 shows. Metallographic observation was realized by use of the light microscope Neophot 21.

3 c) d) Fig. 2 Microstructure image of specimens after heat treatment a) HT1, b) HT2, c) HT3, d) HT Before the actual carrying out the experiment, the roughness values by use of Hommel-Werke T 2000 machine were found, as shown in Fig. 3. Two parameters of each sample sets were monitored according to ČSN EN ISO 287 standard. The R a represents the average surface roughness of the cylindrical and radius sample s part and the R t, represents the deepest micro-notch detected on the cylindrical sample s part, see Figs. and 5. Results of the roughness measuring are shown in Table 2. Fig. 3 Verification of R a and R t values

4 Fig. Micro-notches on cylindrical part Fig. 5 Micro-notches on radius part Table 2 Final values of the surface roughness Set s description R a [μm] R t [μm] Max. Ra μm Max. Ra 0.1 μm Max. Ra 0.05 μm Table 3 Final results of dwell times of specimens Microstructure Max. surface roughness R a [µm] Pressure [kpa] Specified minimum YS (SMYS) Tensile load of SMYS Dwell time [h] ph Solut. Mixed structures with significant banding Mixed structures with mild segregation banding Mixed structures with mild grain size and eliminated segregation banding Mixed structures and ferrite with significant segregation banding 5, 6, 10, 0.1 0, % 8, 3,, , 5,, 6,, 6, , % 2, 1, 2, , 2, 2, 3 3, 2, 1, , % 1, 3, 2, 1 0,05 2, 1, 2, 2 2, 2, 2, , % 3, 2, 2, , 3,, 2

5 After the roughness measurement actual SSC testing started. Specimens were exposed in the solution of distilled water buffered with 0.5 % sodium acetate tri-hydrate and saturated by the hydrogen sulphide. This procedure was carried out according to ISO/DIS standard, which is suitable specially for the SSC testing of pressure vessels and cylinders. The applied load of samples was exactly on the 0.6 of minimal prescribed YS. Start and also finish ph of the corrosion solution was set up exactly to.0 and the pressure in the testing chambers was set up according to above mentioned standard. Final results of dwell times for each specimen of specimens set after the exposure in H 2 S simulating corrosion environment are presented in Table 3. Fig. 6 Fracture surface of specimens after heat treatment a) HT1, b) HT2 Fig. 7 Fracture surface of specimens after heat treatment a) HT3, b) HT

6 Above presented Figs. 6 and 7 represent fracture surfaces with typical cracks distribution and morphology for the SSC exposed samples. Observation was realized by use of the scanning electron microscope SEM JEOL JSM-690 equipped with X-ray analyser EDA. 3. DISCUSSION As it is presented in Table 3, results of values of dwell times do not differ significantly. Micro-notches up to the depth of 2.96 μm incurred during the machining probably do not represent a significant increase in susceptibility to SSC. During the testing, the SSC resistance of high strength version of steel 3CrMo was observed and evaluated. Fracture analysis revealed the typical morphology of the fracture surface, with the extensive network of cracks. Fracture surfaces predominantly show quasi cleavage morphology with lower portion of ductile ridges. The finest fracture facets were observed in case of the HT3, where also the finest microstructure was revealed. In comparison with that mentioned heat treatment mode, fracture surfaces of the HT1 and HT2 were practically similar, even when microstructures showed slight differences as Fig. 2 demonstrates. However, in case of the HT1 more ductile ridges were observed. Fracture surface of the HT showed maximum volume fraction of dimples also thanks to numerous intra-granularly precipitated out carbides as can be seen in Fig. 3b. The initiation of above mentioned cracks could be inter-granular or in area of coarser inclusions, which were detected on basis of Mg-Al-Ca(Mn)S oxi-sulphides. The origin and the distribution of the mentioned cracks was not the topic of this paper but will be target of a future work.. CONCLUSION From perspective of evaluation and the results of specimens dwell times, it is not possible to determine one of three used methods of machining as the most suitable for the SSC specimens machining. The differences between the roughness, 0.1 and 0.05 were observed and evaluated and were not significant. The ISO/DIS standard permits the maximum roughness R a 0.81 μm so the decrease of SSC resistivity can be expected with the more rapid increase of roughness up to the 0.81 μm. This will be the objective of the next work. ACKNOWLEDGEMENT Thanks belong mainly to the Technical director, head of Technical development department and company Vítkovice Cylinders Inc. for the support during the experimental procedures. REFERENCES [1] PRESSUYRE, G., In Int. Congress on Hydrogen and Materials. Paris, 1, 1982, pp [2] SOJKA, J. Odolnost ocelí vůči vodíkové křehkosti. Monograph, VŠB-TU Ostrava, 2007, p [3] CROLET, J.-L. Veus un mécanisme unifié des diverses manifestations de l hydrogène en corrosion aqueuse des aciers. Revue de Métallurgie, 200, 12, pp [] Mazancová, E., Havel, S., Mazanec, K. Influence of Sour Environment on Hydrogen Embrittlement of Steel for Oil Country Tubular Goods (OCTG). Int. J. Pres. Ves. and Piping, 52, 1992, pp