Failure analysis and weld ability improvement of 35%Cr–45%Ni heat resistant alloy

The failure of Ethan cracking tubes in radiant chambers after four years service has been analyzed. The microstructure evolution during solution annealing and its affect on the mechanical properties and repair welding of 35Cr–45Ni heat resistant alloy was studied by means of optical and electron microscopy and by mechanical testing in samples welded both by GTAW and SMAW process. HAZ cracking after welding of cracked or creeped tubes due to formation of continuous brittle carbide network was overcome by localized solution heat treatment. Analysis revealed that dissolution of continuous brittle secondary carbide precipitates promoted an increase in ductility of about 20% which is enough to accommodate the thermal stresses during thermal fluctuations at service.     

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.     

Failure investigation of a convection line elbow

Failure investigation was carried out on a cracked 5″ 304L stainless steel elbow tube for the convection line of an ethylene furnace. The crack was observed in the heat-affected zone (HAZ) at the end leading to the elbow parent material. The crack was along the circumference and was about 12″ in length. The crack started from the toe of the weld to the parent metal on the elbow side. The failure investigation revealed that the HAZ of the elbow tube suffered from stress corrosion cracking and mild grain boundary carbide precipitation during welding. The carbide precipitation was possibly due to improper welding practices such as high heat input and/or extended welding time. The tube thickness, which is relatively thick, was a contributing factor in making proper welding more difficult.     

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.     

Metallurgical Analysis on a Cracked Super Duplex Stainless Steel Flange

A flange-to-pipe weldment in a seawater-intake-return-line suffered from cracking on its flange portion after in service for 4 months. The flange was fabricated from S32750 super duplex stainless steel. The cracked flange was investigated by visual inspection and fractographic examination on the crack surface, metallographic examination, hardness measurement, and chemical composition analysis of the flange-to-pipe weldment. The results of laboratory analysis showed that the flange had cracked in a brittle manner resulting from the presence of large amount of sigma phase in the flange material. Formation of such brittle second phase precipitates in the flange material was due to improper heat treatment.     

Effects of Heat Treatment on Hardness and Dry Wear Properties of a Semi-Solid Processed Fe-27 wt pct Cr-2.9 wt pct C Cast Iron

EfFects of heat treatments on hardness and dry wear properties of a semi-solid processed Fe-26.96 wt pct Cr- 2.91 wt pct C cast iron were studied. Heat treatments included tempering at 500℃, destabilisation at 1075℃ and destabilisation at 1075℃ plus tempering at 500℃, all followed by air cooling. Electron microscopy revealed that, in the as-cast condition, the primary proeutectic austenite was round in shape while the eutectic M7C3 carbide was found as radiating clusters mixed with directional clusters. Tempering did not change the microstructure significantly when observed by scanning or transmission electron microscopy. Destabilisation followed by air cooling led to a precipitation of secondary M23C6 carbide and a transformation of the primary austenite to martensite. Precipitation behaviour is comparable to that observed in the conventionally cast iron. Tempering after destabilisation resulted in a higher amount of secondary carbide precipitation within the tempered martensite in the eutectic structure. Vickers macrohardness and microhardness in the proeutectic zones were measured. Dry wear properties were tested by using a pin-on-disc method. The maximum hardness and the lowest dry wear rate were obtained from the destabilisation-plus-tempering heat treatment due to the precipitation of secondary carbides within the martensite matrix and a possible reduction in the retained austenite.     

Experimental assessment of thermal grain boundary embrittlement after tubeplate failure in a petrochemical heat exchanger

The analysis of multiple cracks in a heat exchanger tubeplate at a petrochemical plant led to experimental replication of in-service damage. High lifting stresses created a first leak that was repaired. Extended branched intergranular cracking developed from the tubeplate surface around the repair weld after few weeks of renewed operation. Chemical analyses failed to detect Na on crack surfaces, while operative conditions also allowed discarding a typical SCC cracking mechanism. Lack of radial interference between tubes and plate, lack of penetration and lack of fusion at tube to tubeplate weld roots were also thought as contributors to cracking. Microstructural analyses revealed martensite clusters in welds and HAZ, and austenite grain boundary precipitates in the tubeplate base material. Thermal cycles were applied to ex-service samples to replicate the conditions for these brittle micro constituents, which were found to be unstable at operating temperatures. Mechanical testing also replicated grain boundary weakness. This inadequate structure was related to welding without proper thermal cycles and heat treatments.     

环氧乙烷精制塔开裂原因分析

通过金相检验、断口分析以及腐蚀产物能谱分析等方法,对某石化公司环氧乙烷精制塔开裂的原因进行了分析。结果表明:材料在焊接过程中,焊接热影响区的组织受到敏化影响,铬的碳化物沿晶界析出,造成该区域贫铬,从而在应力和腐蚀介质的共同作用下,发生了晶间腐蚀开裂。最后提出了相应的预防措施。     

乙二醇不锈钢蒸发器开裂原因分析

采用金相分析、断口微观分析以及腐蚀产物能谱分析等方法，对某石化公司乙二醇不锈钢蒸发器开裂原因进行了分析和研究。结果表明，由于材料在焊接过程中，焊接热影响区的组织受到敏化，铬的碳化物沿晶界呈网状析出，造成该区域贫铬，从而在应力与腐蚀介质的共同作用下，导致设备发生了晶问应力腐蚀开裂。     

Joining of cemented carbides to steel by laser beam welding

Welding of dissimilar materials such as steel and cemented carbides (hardmetals, cermets) is particularly challenging e.g. because mismatches in their thermal expansion coefficients and thermal conductivities result in residual stress formation and because of the formation of brittle intermetallic phases. Laser beam welding of cemented carbides to steel appears as an attractive complementary technique to conventional brazing processes due to its high precision, high process speed, low heat input and the option of welding without filler. Here a laser welding process including pre‐heat treatment and post‐heat treatment was applied successfully to joining as‐sintered and nitrided hardmetals and cermets to low alloyed steel. The microstructure and mechanical properties of the welds are investigated by microscopy, X‐ray diffraction, microhardness measurements, and bending tests. The results reveal that the three‐step laser beam welding process produced crack‐free and non‐porous joints. Nitridation of the cemented carbides results in a significant reduction of the amount of brittle intermetallic phases. The mechanical properties of the joints are competitive to those of the conventional brazed steel‐cemented carbide joints.     

High-temperature degradation mechanism of aluminide coatings on titanium alloys under cyclic oxidation at 750°C

Aluminide coatings prepared on Ti-6Al-4V substrate were able to improve oxidation resistance of the alloy under cyclic oxidation at 750°C both in dry and moist air conditions due to aluminide’s ability to form a stable alumina oxide scale. However, degradation of the coating due to spallation, cracking, internal oxidation and formation of voids with increased cyclic oxidation reduced the lifespan of the coating and the underneath substrate. The main cause of coating degradation for hot-dip specimens is cracks that initiated and propagated perpendicular to the surface. For the plasma spray specimens, the cracks are parallel to the surface. Initiation of cracks in hot-dip coatings are more accredited to residual stresses due to cooling and presence of brittle intermetallic phases TiAl₂ and TiAl. For plasma spray coatings, initiation and propagation of cracks are attributed to presence of entrapped oxides, pores and grain boundaries of the deposited splats whose flattened edges are parallel to the surface of the coating. Presence of water vapor, too, acts as an oxygen carrier and thus promotes oxidation internally, inhibits growth of continuous protective alumina oxide scales and weakens the scale/alloy interfacial toughness. Water vapor therefore accelerates degradation by increasing spallation and cracking rate of the coating.     

An investigation on thermal, metallurgical and mechanical states in weld cracking of Inconel 738LC superalloy

The influence of various conditions of Inconel 738 superalloy welding or deposition welding has been studied in order to shed light on the coupling between thermal, metallurgical and mechanical states in the heat affected zone (HAZ) in which cracking may occur particularly during welding and post-weld heat treatment. Predominant crack controlling factors have been highlighted thanks to different pre-weld and post-weld heat treatments, in addition to various welding rates and pre-heating prior to welding. These factors are mainly the material ductility related to the morphology and volume fraction of intermetallic precipitates and thermally induced residual stress. It has appeared that reducing thermally induced residual stress could be more effective for preventing cracking than controlling the material ductility in the related zones thanks to adjustment of pre-weld and post-weld heat treatments. With the objective of cracking remediation, welding on preheated parts leads to lower weld power, to reduce significantly thermal gradients, decreases thermally induced stress and impedes cracks formation despite some localized and temporary decrease in alloy ductility.     

Laser welding of Ti–6Al–4V to Nitinol

Formation of brittle intermetallic phases in addition to different thermal expansion coefficients associated with dissimilar welding leads to the formation of transverse cracks in weld metal and eventually restricts widespread applications of dissimilar joints. Therefore, joining technology should be expanded in field of dissimilar welding in order to solve its difficulties. In the present study, an experimental work with pulsed Nd:YAG laser was performed for dissimilar welding of Ti–6Al–4V and Nitinol. Autogenous welding of these two alloys resulted in joints with poor strength and ductility due to the formation of transverse cracks in the weld metal. Therefore, the chemical composition of the weld metal has to be modified in order to reduce the formation of brittle phases and eliminate subsequent cracking. In this work, this was done by insertion of a copper interlayer with a thickness of 75 μm between the base metals. The results indicated that insertion of copper interlayer has a great influence on the reduction of the amount of Ti₂Ni brittle intermetallic phase, elimination of transverse cracks through the weld metal and eventually improvement of mechanical properties of the joints. Insertion of copper interlayer was very useful since it altered the cracked autogenous joint to a joint which could withstand a tensile stress of 300 MPa.     

Post-weld heat treatment cracking in autogenous GTA welded cast Inconel 738LC superalloy

Abstract

The post-weld heat treatment (PWHT) cracking in autogenous gas tungsten arc (GTA) welded Inconel 738LC superalloy, which was given two different preweld heat treatments, was studied. One of the preweld heat treatments, designated as SHT, consisted of solution heat treatment at 1120°C for 2 h in vacuum followed by argon quenching. The second preweld heat treatment, designated as UMT, consisted of solution treatment at 1120°C for 2 h followed by air cooling and then aging at 1025°C for 16 h followed by water quenching. The welded specimens were given the same conventional PWHT, which consisted of SHT at 1120°C for 2 h in vacuum followed by argon quenching and subsequent aging at 845°C for 24 h in vacuum. Microstructural examination of the welded SHT and UMT treated material showed that intergranular microfissuring occurred during welding only in the heat affected zone (HAZ) with some cracks extending into the adjoining base metal (BM), whereas after the PWHT microfissures were observed in the fusion zone (FZ), HAZ and the BM far removed from the HAZ. The crack width ranged from 5 to 10 μm in the PWHT specimens as compared with 1–2 μm in the as welded sections. Although similar type of cracks was observed in samples given the two preweld heat treatments, the UMT preweld heat treatment was found to result in a significant reduction in average total crack length and average crack length, both during welding and during the subsequent PWHT. After PWHT, SHT samples had ～43% more cracking than the UMT samples. It is suggested that a larger particle size of γ′ precipitates in the HAZ and a smaller size of HAZ in the as welded samples, combined with a softer BM of the UMT material (hardness 280 ± 12 HV10, as compared with 380 ± 10 HV10 of the SHT material) resulted in an improved capability of the material to absorb the strain–aging stresses, and hence a reduced incidence of cracking during PWHT.     

WC含量对Ni3Al-WC复合材料组织的影响

以WC，NiAl,NiB和Ni粉等球磨混合粉末烧结制备Ni3Al-WC复合材料焊条，用氩弧焊堆焊在1Cr25Ni20Si2耐热钢的表面形成Ni3Al-WC复合材料。当WC的含量从5%提高到30%时，在焊接过程中，由于熔池中溶解碳含量的增加，保护能力不断增强，溶池中的Al由部分氧化至完全不氧化，冷却后复合材料的组织也从碳化物包裹的氧化物/金属间化合物转变成碳化物/金属间化合物，获得无裂纹的焊接表面。     

The effects of pre- and post-weld heat treatment variables on the strain-age cracking in welded Rene 41 components

Rene 41, a precipitation-hardened nickel-base superalloy, is mostly used for hot section parts of jet aircraft engines. When cracks occur in these parts, welding is frequently utilized for crack repair to extend their service life. Rene 41 is very susceptible to strain-age cracking during welding and post-weld heat treatment because of its strengthening mechanism. The strengthening mechanism depends on the existence of gamma prime (γ′) phase and the morphology of carbides both of which are affected by heat treatment. In this study, the effects of pre- and post-weld heat treatment variables such as heating rate, holding temperature and time, and cooling rate on the strain-age cracking of Rene 41 during welding and post-weld heat treatment were investigated. An optimum combination of heat treatment variables was investigated to obtain a microstructure that would be less susceptible to strain-age cracking during welding and post-weld heat treatment. Pre- and post-weld solution heat treatments at 1080 °C for 30 min with 20 °C/min heating and 34 °C/min cooling rates produced optimum microstructures, and no cracks were observed in the Rene 41 samples after welding. MC carbides were dissolved lightly and distributed homogenously within the grains. M₆C carbides precipitated as globules within the grains and in the grain boundaries. M₂₃C₆ carbides precipitated as chains of globules in the grain boundaries.     

Failure analysis of TP304H tubes in the superheated steam section of a reformer furnace

The cracking failure of TP304H tubes in the superheated steam section of a reformer furnace was analyzed. Through the analysis of macro-appearance, micro-appearance of specimens with cracks, metallurgical structure of specimens from an intact pipe section and cracked pipe section, energy spectrum detection of fracture surface, residual stress measurement, and investigation of the service medium, the cracking mode was described as the stress corrosion cracking (SCC) of austenitic stainless steel. In this case, the materials in the heat affected zone were sensitized by inappropriate welding technology. This together with the higher pH value in the steam due to the failure of a gas-liquid separator led to the final cracking of the reformer furnace tube. So the inappropriate welding technology and the failure of the gas-liquid separator were the main factors in this fracture accident.     

An experimental study of the residual stresses,and their alleviation,in tube to tube-sheet welds of industrial boilers

Micro-crack development in welds, including repair welding, of the tube to tube-sheet region of industrial boilers is not an uncommon problem. Often such cracking is exacerbated by stress corrosion cracking or thermal fatigue, in which the residual stresses associated with welding can play a major role. In this paper, a custom built, but robust, air abrasive centre hole drilling facility was used; which induced negligible machining stresses. This system was modified to attach to the (vertical) tube sheets of boilers on site, and to sample residual stresses in the tube to tube-sheet welds, of both conventional circumferential fillet welds of protruding tubes as well as recessed ‘J’ configuration type. The experimental test programme evaluated transverse and circumferential profiles of the residual stresses. Global post-weld heat treatment (PWHT) of a whole boiler, indicated substantial (75%) reductions in the tensile peak residual stress. A localised PWHT technique, using a 1 kW incandescent light source inserted into the tube, easily achieved the requisite temperatures and heating rates, but led to significant increases in local residual stresses. The implications of such localised PWHT techniques on residual stress and consequent service life, are therefore of vital importance with regard to plant integrity and safety.     

Effect of stress relief at 350 °C and 550 °C on the impact properties of duplex stainless steels

The production of massive duplex stainless steel castings weighting over 2 t, with thicknesses exceeding 5 in. represents a major challenge for the foundry industry. The difficulty in manufacturing such castings lies in the fact that thick sections experiment low cooling rates during the solidification process and during the solution annealing and water quenching heat treatment.As a result, intermetallic phases such as sigma phase (σ), Chi phase (χ), G phase, R phase, and complex carbides may precipitate, causing the material to be extremely brittle [Martins M, Casteletti LC. Effect of heat treatment on the mechanical properties of ASTM A890 grade 6A super duplex stainless steel. J ASTM Int 2005;2(1) [January]. [1]].After solution annealing and water quenching, the steel is, in principle, free of intermetallic precipitates, but will contain residual stresses resulting from rapid cooling on quenching. During and after machining, these stresses may produce dimensional distortions in the casting, which can be avoided or at least reduced with stress relief heat treatments at intermediary temperatures, taking care to prevent the loss of mechanical properties, mainly impact toughness.The purpose of this study was to investigate the behavior of CD4MCu and CD4MCuN duplex stainless steels in impact tests under the conditions of solution annealing and water quenching and stress relief at 350 °C for 4 h and at 550 °C for 2 h. Compared to CD4MCu the high nitrogen content of CD4MCuN stainless steel has a more balanced microstructure with similar ferrite and austenite contents, providing it with higher energy-absorbing capacity in impact tests. CD4MCuN fracture surfaces have predominantly fibrous structures typical of high toughness materials, while the CD4MCu steel's fracture surface shows cleavage facets typical of low toughness materials. The stress relief heat treatments reduced the impact toughness of the CD4MCu alloy but did not affect the CD4MCuN alloy.     

Joining of γ-TiAl to low alloy steel by electron beam welding

Abstract

The weldability of cast γ-TiAl with a low alloy steel by electron beam welding was investigated in this study. It was found that when a higher heat input was employed, solid state cracking appeared in the welds due to a high thermal stress, as well as formation of a brittle TiC phase and a small fraction of Ti₃Al intermetallic. Crack free welding could be achieved if welding parameters were properly controlled such that thermal stress was released and formation of TiC and Ti₃Al was minimised.