Stress corrosion cracking of stainless steel bellows of satellite launch vehicle propellant tank assembly

Bellows made of austenitic stainless steel (AISI 304 grade) are being used as a conduit for liquid fuel and oxidizer in the propellant tank of a satellite launch vehicle. A few bellows were found leaky during re-pressure tests after 6 years of storage. A number of cracks were found originating from weld fusion lines. One of the leaking bellows was subjected to detailed metallurgical and chemical analysis. The synergistic effect of chloride ions and thermal stresses from welding was identified as the cause-a typical example of stress corrosion cracking (SCC).     

Metallographic analysis of embedded crack in electron beam welded austenitic stainless steel chemical storage tank

An AISI 304L austenitic stainless steel tank used for chemical storage showed cracks during the post weld qualification programme. A crack of 75 mm length embedded within the weld pool was subjected to detailed metallographic analysis. The results revealed that the cracking was due to a shift in the solidification mode from primary ferrite to primary austenite. The residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under fixtured condition, are additional factors which caused cracking.     

Metallurgical investigation on stainless steel bellows used in satellite launch vehicle

Bellows made of austenitic stainless steel AISI 304 grade are being used as conduit for liquid fuel and oxidizer in propellant tank of satellite launch vehicle. These bellows encountered frequent leakage problems. Leakage locations were found to be along the fusion line of ring to ply weld. In many of such failures, synergistic effect of chloride ions and thermal stresses from welding was the cause identified. Detailed metallurgical characterization of bellows with different weld parameters have been carried out, which served as an efficient tool for qualification of the processing of bellows. This paper highlights various metallurgical features observed in stainless steel bellows during process qualification programme and their impact on the performance.     

Failure investigation of 321 stainless steel pipe to flange weld joint

Failure investigation was conducted on a refinery pipe-to-flange weld joint that suffered cracking. Both the pipe and flange are made of AISI 321 stainless steel. The flange was circumferentially welded to the pipe which is seam welded. The investigation revealed that both the circumferential and seam welds were in sound conditions, namely no evidences of sensitization, lack of weld penetration, and voids or porosities. Thus, welding practices were not suspected to be the cause of failure. The failure of the weld joint was found to have started at the δ-ferrite phase in the flange material and propagated through the circumferential and seam welds. The failure mode was concluded to be chloride stress corrosion cracking synergized by the presence of H₂S. The presence of corrosive compounds in the refinery stream and the residual stresses at the weld joint triggered active anodic dissolution of the δ-ferrite precipitates, resulting in cracking of the material.     

Finite element analysis of the effect of welding heat input and layer number on residual stress in repair welds for a stainless steel clad plate

Stainless steel clad plate is widely used in petroleum, chemical and medicine industries due to its good corrosion resistance and high strength. But cracks are often formed in clad layer during the manufacture or service, which are often repaired by repair welding. In order to ensure the structure integrity, the effects of residual stress need to be considered. The objective of this paper is to estimate the residual stress and deformation in the repair weld of a stainless steel clad plate by finite element method. The effects of heat input and welding layer number on residual stresses and deformation have been studied. The results show that large residual stresses have been generated in the repair weld. The heat input and layer number have great effects on residual stress distribution. With the heat input and welding layer number increasing, the residual stresses are decreased. Using multiple-layer welding and higher heat input can be useful to decrease the residual stress, which provides a reference for optimizing the repair welding technology of this stainless steel clad plate.     

Hot cracking susceptibility of boron modified AISI 304 austenitic stainless steel welds

Abstract

The hot cracking susceptibility of welds made on AISI 304 stainless steel modified with from 0·2 to 1·0 wt-%B has been investigated. Varestraint tests showed that the hot cracking susceptibility is high for boron additions of about 0·2%, but is decreased when the boron content is increased to ≥0·5%. Steels containing about 0·2%B were found to have a wide solidification temperature range and their high temperature ductility was low compared with boron free AISI 304 steel and the other boron modified steels. Ferrite precipitation was inhibited in the 0·2%B steels and the formation of low melting point grain boundary films was thereby promoted. Increasing the boron content to ≥0·6% reduces the coefficient of thermal expansion and narrows the solidification temperature range. In addition, crack refilling was observed, resulting in improved hot ductility and high resistance to hot cracking. It is concluded that in structures where weld restraint forces are not high, hot cracking is not likely to occur if boron additions of >0·6% are made to AISI 304 stainless steel. In T-type and Fisco weld cracking tests, in which the weld restraint forces are close to those experienced by actual structural welds, the boron modified stainless steels show a low hot cracking susceptibility which is not significantly different from that of boron free AISI 304 steel.

MST/1548     

304不锈钢局部干法水下激光填丝焊接工艺及焊缝性能研究

目的 采用自主研制的水下激光填丝焊接装备,在304奥氏体不锈钢板材表面进行U形坡口激光填丝焊接试验,为304不锈钢水下修复工作提供技术参考。方法 在功率为5 600 W、焊接速度为6 mm/s、送丝速度为205 cm/min、保护气体流量为15 L/min、排水气体流量为30 L/min的条件下进行焊接试验,并对空气和水下环境下的焊缝进行对比检测分析。通过光学显微镜分析2种环境下焊缝的显微组织;对2种焊缝进行拉伸、弯曲等力学性能测试;采用显微硬度计测试1 kg载荷下不同区域的显微硬度;使用VersaSTAT3F电化学工作站测定在3.5%（质量分数）的NaCl溶液中2种焊缝的开路电位和极化曲线。结果 2种环境下的焊缝均无明显裂纹、气孔等缺陷;显微组织主要由奥氏体和铁素体组成,但2种环境下焊缝的奥氏体晶粒大小和铁素体形状均略有差别,焊缝拉伸断口均为典型的韧性断裂形貌且抗拉强度符合304不锈钢标准。2种环境下焊缝的微观组织和晶粒大小不同,水下焊缝硬度高于空气的。通过分析2种环境下焊缝的开路电位和极化曲线,可知水下焊缝的耐腐蚀性略高。结论 所开发的局部干法水下激光填丝焊接工艺可以满足实际工程中...     

Considerations for the weldability of types 304L and 316L stainless steel

The susceptibility of austenitic stainless steels to the formation of two distinct weld defects, solidification cracking and lack of penetration, is related to the chemical composition of the base and filler material. The propensity for cracking is determined primarily by the solidification mode and the amount of residual tramp elements such as phosphorous and sulfur. High sulfur levels can lead to weld centerline cracking and heat affected zone (HAZ) cracking while very low sulfur levels (less than ∼50 ppm) in types 304L and 316L are associated with lack of penetration weld defects and a distinct loss in puddle control during fusion welding. A calculated Cr_eq to Ni_eq ratio of 1.52 to 1.9 is recommended to control the primary mode of solidification and prevent solidification cracks in type 304L while the Cr_eq/Ni_eq ratio of 1.42 to 1.9 is recommended for type 316L stainless steel. A lower limit of 50 ppm sulfur is recommended to avoid possible lack of penetration. These ranges should be validated by welding trials for specific weld processes and applications.     

Microstructure of copper–AISI type 304L electron beam welded alloy

Abstract

Heterogeneous butt welding of copper and AISI type 304L stainless steel was carried out using the electron beam process. Examination by scanning and transmission electron microscopy has indicated the possibility of obtaining joints free of cracks and porosity. Energy dispersive microanalysis of the weld bead cross-section has demonstrated the presence of non-equilibrium phases. The results show that the binary Cu-Fe equilibrium diagram is unable to predict the weld microstructure even at the moderate cooling and solidification rates expected under the present welding conditions. The feasibility of the Cu-304L electron beam welding process is therefore hindered by the problem of microstructural stability of the joint because of possible phase transitions during the service life of welded components.     

Impact behavior of different stainless steel weldments at low temperatures

A comparative study was made of the fracture behavior of austenitic and duplex stainless steel weldments at cryogenic temperatures by impact testing. The investigated materials were two austenitic (304L and 316L) and one duplex (2505) stainless steel weldments. Shielded metal arc welding (SMAW) and tungsten inert gas welding (TIG) were employed as joining techniques. Instrumented impact testing was performed between room and liquid nitrogen (?196 °C) test temperatures. The results showed a slight decrease in the impact energy of the 304L and 316L base metals with decreasing test temperature. However, their corresponding SMAW and TIG weld metals displayed much greater drop in their impact energy values. A remarkable decrease (higher than 95%) was observed for the duplex stainless steel base and weld metals impact energy with apparent ductile to brittle transition behavior. Examination of fracture surface of tested specimens revealed complete ductile fracture morphology for the austenitic base and weld metals characterized by wide and narrow deep and shallow dimples. On the contrary, the duplex stainless steel base and weld metals fracture surface displayed complete brittle fracture morphology with extended large and small stepped cleavage facets. The ductile and brittle fracture behavior of both austenitic and duplex stainless steels was supplemented by the instrumented load–time traces. The distinct variation in the behavior of the two stainless steel categories was discussed in light of the main parameters that control the deformation mechanisms of stainless steels at low temperatures; stacking fault energy, strain induced martensite transformation and delta ferrite phase deformation.     

Failure Analysis of HAZ Cracking in Low C–CrMoV Steel Weldment

This case study describes the failure analysis of steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low C–CrMoV alloy steel that was subsequently single pass butt welded using gas tungsten arc welding. No cracks were found in visual inspection of the welds; however, X-ray radiography showed small discontinuous cracks on the surface in the area adjacent to weld bead, i.e. heat affected zone. The welding of nozzle parts made of same material was a routine process and this type of cracking did not occur in the past. Therefore, it became essential to determine the root cause of the failure. A detailed investigation including visual examination, non-destructive testing, optical microscopy, microhardness measurements and residual stress measurements were carried out to find out the primary cause of failure and to identify actions required to avoid its reoccurrence in future. Results of the investigation revealed that the principal cause of failure was the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in the high transformation stresses. These transformation stresses combined with the thermal stresses and the constraint conditions to cause intergranular brittle fracture.     

Mechanical and technological analysis of AISI 304 butt joints welded with capacitor discharge process

In the present work, the capacitor discharge welding process (CDW) applied on AISI 304 circular bars was studied. The CDW process is essentially an electrical resistance welding technology, realized through current pulses of high intensity and discharged by large capacitors; the process allows to reduce stress concentration effects at the weld toe, obtaining thin welds and achieve good material integrity.CDW process characteristics lead to conceive the idea to investigate on the interaction between the weld technological aspects and the related mechanical properties.In this research activity, 150 cylindrical specimens with bore diameter 6 mm and different igniter dimensions were machined in AISI 304 austenitic stainless steel; a special equipment was designed to clamp specimens and to assure perfect electrical continuity.The main CDW welding parameters (energy input P, applied forces and igniter dimensions) were studied in order to optimise the welding process. The static and fatigue properties were finally analysed for the welded bars and the results were correlated to process parameters; mechanical tests give good results with respect to base metal if the proper welding parameters are used, despite the fact a brittle character was observed for the welded joints.     

Lamellar Tearing: A Failure Case Study

A variety of materials are used in the Oil & Gas industry ranging from carbon steel, stainless steel to nickel alloys, etc. including non-metallic materials as well. Amongst these, carbon steel is the industry favorite because of many of its desirable attributes like machinability, weldability, availability, and cost. Wide use of carbon steel in the industry is also due to the presence of required properties for the specific application. In spite of carbon steel’s wide uses, poor workmanship, improper heat treatment, or negligent manufacturing processes can reduce its service life and also can lead to unexpected failures. One such failure case involves carbon steel plate meant for construction of a tank that failed by cracking immediately after the welding operation. A detailed investigation was carried out by means of visual examination, metallographic and chemical analysis, SEM, and EDAX analysis. Microstructure revealed banded pearlitic structure along with a number of inclusions, a few of which were elongated. In conclusion, stresses due to weld joint restraint, elongated inclusions, and high transverse direction stresses after welding resulted in lamellar tearing which propagated in the linear direction along the weld line.     

Failure Analysis of Boat Rudder Fitting Fabricated from a Type 304 Stainless Steel

The failure of a type 304 stainless steel component subassembled by welding and used on a boat in a marine environment was investigated. The mechanism of damage initiation and the cause of final failure were investigated. Initial examination of the component indicated deep branching cracks that were thought to have developed during service. The combination of microhardness test and finite element modeling (FEM) was employed to probe micromechanical properties of the damaged area. The corrosion observed was followed with scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). FEM analysis suggests that the cracked area had been subjected to tensile stresses in service. Microhardness across the welded section did not show any mechanical degradation across the weld and heat-affected zone. The cracked area was evidently corroded, and the microanalysis in the SEM/EDS indicated the presence of corrosion products. Regions around the cracks especially at the root of the crack were found to be severely depleted of Ni. It is evident that the primary course of failure was from the cracking from SCC attack and that the pitting observed is a secondary effect in the cracked region.     

Effect of buttering and hardfacing on ballistic performance of shielded metal arc welded armour steel joints

Quenched and tempered (Q & T) steel closely confirming to AISI 4340 is well known for its superior ballistic performance and hence used in the fabrication of armour vehicles. These steels are traditionally welded by austenitic stainless steel (ASS) fillers to prevent hydrogen induced cracking. Due to weld thermal cycles and under matching fillers, the armour steel joints show poor ballistic performance compared to the base metal. Attempts were made to deposit hardfaced interlayer between ASS weld metals. Though this method yielded marginal improvements in ballistic performance, cracks were observed in between base metal and hardfaced layers. In this investigation an attempt has been made to eliminate these cracks by depositing a soft buttering layer using ASS consumable in between base metal and hardfaced layer. This paper reveals the effect of buttering and hardfacing on ballistic performance of shielded metal arc welded armour steel joints.     

Characterization of Weld Solidification Cracking in a Duplex Stainless Steel

Weld solidification cracking in the duplex stainless steel SAF 2205 has been investigated and compared with that of alternate duplex and austenitic stainless steels. Varestraint weld-ability testing showed SAF 2205 to exhibit a lower cracking susceptibility than that of the duplex stainless steel Ferralium 255 but greater than that of a Type 304 austenitic stainless steel which solidified as ferrite and exhibited Ferrite Number 8 (FN 8) in the weld fusion zone. The high augmented strain levels required to induce cracking in these three alloys during Varestraint testing indicated a high resistance to solidification cracking at strain levels normally encountered in structural weldments. Cracking susceptibilities of the duplex and Type 304/FN-8 stainless steels were appreciably lower than that of a Type 304L stainless steel which solidified entirely to austenite and exhibited less than FN 1 in the weld fusion zone.

Microstructural characterization of SAF 2205 using conventional black-and-white and two different color metallography techniques showed solidification cracks to be associated with ferrite grain boundaries. Color metallography was also effective in revealing the fusion zone solidification structure and delineating second phases, including inter- and intragranular austenite and fine Cr₂N precipitates. Fractographic analysis of solidification crack surfaces from SAF 2205 Varestraint samples revealed dendritic and flat topographies, and confirmed a solidification versus solid-state cracking mechanism.     

Softened zone formation and joint strength properties in dissimilar friction welds

The mechanical properties of dissimilar MMC/AISI 304 stainless steel friction welds with and without silver interlayers were examined. The notch tensile strengths of MMC/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds increased when high friction pressures were applied during the joining operation. The higher notch tensile strengths of dissimilar MMC/AISI and MMC/Ag/AISI 304 stainless steel friction welds resulted from the formation of narrow softened zones in MMC material immediately adjacent to the bondline. The influence of softened zone width and hardness (yield strength) on the notch tensile strengths of dissimilar welds was analysed using finite element modelling (FEM). FEM in combination with the assumption of a ductile failure criterion was used to calculate the notch tensile strengths of dissimilar joints. The key assumption in this work is that dissimilar weld failure wholly depended on the characteristics (mechanical properties and dimensions) of the softened zone formed in MMC material immediately adjacent to the bondline. The modelling results produced based on this assumption closely correspond with the actual notch tensile strengths of dissimilar MMC/Ag/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds.     

Analysis of sensitization phenomenon in friction stir welded 304 stainless steel

This study evaluates the degree of sensitization (DOS) of 304 stainless steel joined by friction stir welding (FSW). Single-loop electrochemical potentiokinetic reactivation tests were performed using a 0.5 mol/L H₂SO₄ + 0.01 mol/L KSCN solution. Sensitization was promoted by exposition of the stainless steel at temperatures between 400°C and 850°C. The microstructure was characterized using optical microscopy to identify the weld zone and the base metal. The samples treated at 550°C showed the most severe intergranular corrosion. The DOS was lower in the weld zone than in the base metal after heat treatments. This reduction in the DOS for the weld zone indicates that FSW is a beneficial process in joining stainless steel.     

The edge-cracking of AISI 304 stainless steel during hot-rolling

The hot-rolled plates of AISI 304 stainless steel, containing edge cracks of different intensities, were examined. The austenitic matrix of the steel contained small amounts of ferrite inhomogeneously distributed across the width and the thickness of the plate. A correlation was found between ferrite content and edge cracking: the higher the ferrite content the longer the edge cracks. Among the chemical elements present in the steel, the most critical effect on ferrite content was exerted by carbon and nitrogen. The longest edge cracks were observed for plates with the lowest content of carbon and nitrogen. A possible contribution of steel chemistry and heating temperature to changes in the steel phase composition and the probability of edge cracking is discussed.     

Turbine Shroud Cracking Investigation and Repair

Outer shroud segments fabricated from a cast 310 stainless steel were found to have cracked following extended service. Several of these cracked shrouds were sent to the GE-Poland Materials Laboratory for metallurgical investigation with a view to determine the cause of cracking. Detailed investigation revealed cracking to be associated with a sigma phase, a brittle TCP intermetallic, which had developed over time during engine operation. Initial attempts to weld repair the cracks proved unsuccessful as cracks were discovered both during and after the weld repair procedure. A new weld repair procedure incorporating a pre-weld solution annealing heat treatment to remove the sigma phase before welding was successfully developed, thus alleviating the cracking concern.