Corrosion behaviour of a thin section martensitic stainless steel GTA weldment in chloride solutions

A thin section martensitic stainless steel was welded by gas tungsten arc welding and characterized for the microstructure, hardness and corrosion behaviour in chloride solutions. Welds free from defects could be produced by autogenous welding under the optimized welding conditions. The weld metal was over-matched in terms of mechanical properties (hardness, tensile strength). The general corrosion resistance and the passivation behaviour of the weld metal/heat affected zone (HAZ) region were on par with that of the parent material in chloride and sulphuric acid test electrolytes; however, in terms of pitting corrosion resistance, the martensitic-structured weld metal/HAZ region was marginally inferior compared to its parent material.     

Growth mechanism of compound at interface of Ti-6Al-4V joint brazed with 72Ag-28Cu filler alloy

Abstract

The growth process of Ti-Cu compound at the interface of a Ti-6Al-4V/72Ag-28Cu (wt-%) joint was analysed using X-ray diffraction, SEM, and energy diffraction spectra. According to the investigated results, when the joint was brazed for a relatively short holding time, atoms of Ti and Cu diffused into the interface would combine into Ti₂Cu by eutectoid reaction during the cooling stage. As the holding time is beyond the critical brazing time, Ti₂Cu compound decomposed owing to a large amount of Ti in the base metal dissolving into the brazing zone and the relatively gentle concentration gradient of Cu, thus resulted in the solid dissolving of Cu into Ti. In this case, the resulting joints exhibited high strengths. On the basis of the analysis mentioned above, a concept 'critical brazing time' was proposed.     

循环乙烷裂解炉炉管失效分析

某材料为35Cr45Ni高镍铬奥氏体耐热钢的循环乙烷裂解炉炉管在停炉后发生开裂,通过对其进行宏观形貌、微观形貌、显微组织以及化学成分等方面的分析和理论计算,找出了炉管开裂失效的原因,并提出了相应的预防措施和建议。结果表明:炉管长期在高温下服役,导致其强度降低,塑性和韧性变差;同时炉管内形成了很厚的结焦层,由于结焦层的热膨胀系数和炉管的热膨胀系数相差很大,使其在停炉冷却过程中对炉管内壁产生了很大的环向应力,当其超过炉管的抗拉强度时,就会导致炉管发生纵向全管开裂。     

Failure Analysis of a Vehicle Engine Crankshaft

An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.     

Functional properties of a spark plasma sintered ultrafine-grained 316L steel

A micrometric austenitic stainless steel 316L powder was densified by spark plasma sintering. The process parameters were varied over wide ranges and the impact of such variations on sintered materials was studied through the characterization of their microstructures, densities, hardness and corrosion resistance. For comparison with the properties of traditionally cast 316L, all these investigations were also systematically carried out on as cast samples. The sintered stainless steel produced this way was highly densified, with grains of a micrometric size and the forming process did not induce any residual strain gradients as shown by transmission electronic microscopy analysis. The investigation of the corresponding mechanical properties reveals an enhancement of hardness up to twice the value measured on one sample of as cast 316L. This result is in good agreement with the Hall–Petch formalism. Additionally, in the matter of corrosion behavior, fully dense samples display an enhanced passive state in chloride media compared to as cast material. Spark plasma sintering appears to be an interesting alternative elaboration way of ultrafine 316L stainless steel giving materials with high stress resistance, without strain gradients through the volume, and promising functional properties concerning corrosion behavior.     

Relationship between microstructure,hardness and corrosion resistance in 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons

The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M₇C₃ with stoichiometry as (Cr_3.37, Fe_3.63)C₃, (Cr_4.75, Fe_2.25)C₃ and (Cr_5.55, Fe_1.45)C₃, respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M₇C₃ in 20 wt.%Cr iron, whereas both M₂₃C₆ and M₇C₃ secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.     

Failure analysis of styrene reactor tubes

The analysis of failure of the styrene reactor tubes made of stainless steel has been reported where the tubes contained catalyst as well as ethyl benzene in vapour form along with superheated steam at 570 °C. The tubes were exposed to flue gases causing heating of tubes to 790 °C in the upper part where cracking and failure were mostly noticed. The studies included microstructural examinations of cracked and uncracked tubes, fracture surface investigations and estimation of creep rupture strength, etc. The degradation in microstructure such as extensive grain coarsening, sigma phase formation, carbide formation along grain boundaries, etc., led to development of cracks/voids within the grains and along the grain boundaries and also excessive oxidation of tubes. Possible chemical attack by the feed on the inner surface of tubes further deteriorated the tube life. Suggestions for avoidance of failure have been listed.     

A new resource-saving,low chromium and low nickel duplex stainless steel 15Cr–xAl–2Ni–yMn

A new family of resource-saving, low Cr and low Ni duplex stainless steels, with compositions of 15Cr–xAl–2Ni–yMn (x = 1.2–2.8, y = 8–12, wt.%) has been developed by examining the effect of Al and Mn on microstructure, mechanical property and corrosion property. The results show that 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn alloys have a balanced ferrite–austenite relation and that 15Cr–2.8Al–2.0Ni–12Mn alloy has a primary ferrite phase structure. The ferrite volume fraction increases with the solution treatment temperature and Al content while decreases with Mn content. No precipitate was found after solution-treated at 750 °C for 30 min. 15Cr–1.2Al–2.0Ni–8Mn alloy has a strong strain hardening effect, and 15Cr–2.0Al–2.0Ni–10Mn alloy has a good TRIP effect. Both of the 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn alloys have excellent impact toughness at low temperature with the impact energy higher than 125 J at −40 °C. The pitting corrosions always occur in austenite phase. Among the designed alloys, 15Cr–1.2Al–2.0Ni–8Mn and 15Cr–2.0Al–2.0Ni–10Mn are found to be excellent alloys with a proper phase proportion and a better combination of superior mechanical property and good pitting corrosion resistance.     

Failure Investigation of Cooling Plates of a Blast Furnace

Premature failure of cooling plates of a blast furnace in service has been investigated. The cooling plates are water-cooled copper channels shielded by refractory lining. Industrial cooling water (ICW) flows at 5 bar pressure through the cooling plates. The investigation consisted of visual inspection, chemical analysis, microstructural examination by optical microscopy, energy-dispersive spectroscopy (EDS), and hardness measurement. Visual inspection revealed that the refractory lining on the component was damaged, thereby exposing the cooling plates to elevated temperatures of the furnace atmosphere. This led to overheating of the cooling plates. Chemical analysis revealed that the component was essentially pure copper (~99.24% purity). Microstructural analysis showed grain size variation across the section of the component. Also, annealing twins were revealed on optical examination. A through-thickness pit was identified on the failed component during the optical examination. The examination also revealed branched cracks associated with scale in the component. Energy-dispersive spectroscopy (EDS) confirmed the scale to be oxides indicating high-temperature oxidation of the component resulting in pitting. Analysis suggested overheating due to damage of the refractory lining as the prime cause of failure of the component.     

某热电厂炉管开裂失效分析

通过对某热电厂开裂失效的炉管进行宏观检验、化学成分分析、力学性能试验、金相检验等分析,确认了引起炉管开裂的原因。结果表明:炉管开裂主要是由于高温环境长期运行导致炉管材料发生蠕变,力学性能下降所致。建议应根据规范要求采取加强炉管服役期间检修时的监测工作,按规范要求在锅炉高温部位设置蠕胀监测点,将炉管的服役期控制在总的相对蠕变应变小于2%之内,以保证锅炉的正常安全运行。     

Failure Analysis of Corroded Tube–Tubesheet in an Oxy-chlorination Reactor

After the replacement of a 20-year-old oxy-chlorination reactor, the top tube–tubesheet of the newly installed reactor was seriously corroded and lasted for only 40 days. Samples extracted from the retired and new tube–tubesheet were examined to identify the possible causes for the failures. Due to the corrosion, the sealing design (tube expansion followed by seam weld) in the tube–tubesheet was not a successful methodology to prevent leakage in this chemical process. However, this short-term breakdown is attributed to the operating conditions (operating pressure and service temperature) in this petrochemical refinery, because no major discrepancy existed in the material characterization of the new and retired top tube–tubesheet.     

Investigation of Premature Failure of a Coal-Fired Boiler Superheater Tube

Journal of Failure Analysis and Prevention - A metallurgical investigation has been carried out to ascertain the cause of premature failure of boiler superheater tube. The tube material was...     

Influence of Mandrel on the Performance of Titanium Tube with Cold Rotary Swaging

Rotary swaging is a process to reduce the diameter of tubes and/or rods. It can be used to produce tubes or rods with a variety of external and internal diameters and shapes. Mandrel is an important part of the forming process. In this work, the influence of mandrel on tube performance is discussed. Data show that larger mandrels provide better forming results. Wall thickness standard deviation can be reduced from 0.108 (without mandrel) to 0.031 (7.4 mm diameter) and the hardness of the tube can be increased from 273.5 HV (original material) to 328.5 HV (7.4 mm diameter). Surface toughness can also be improved. Analysis shows that the microstructure is distinctly different from that of the original material and it varies by changing the mandrel. This research validates the use of cold swaging as an efficient method of forming titanium.     

3种船用钢在南海海洋环境中8年的腐蚀行为

研究了３Ｃ（Ｗ ）、１６Ｍｎ、９２１ 三种钢在南海榆林海域全浸、潮差、飞溅环境中暴露８ 年的腐蚀行为。其中,９２１ 钢在飞溅区具有优良的耐蚀性,而在全浸和潮差区耐蚀性不如３Ｃ（Ｗ）钢和１６Ｍｎ 钢的。发现９２１ 钢在全浸区暴露２ 年后发生腐蚀率随时间增加而增大的“逆转”现象。     

Investigation of Fatigue Failure of Roll Shafts in a Tube Manufacturing Line

A failure analysis of steel shafts operating as roll components in a tube drawing machine was conducted. Visual inspection, optical microscopy, SEM fractography, and hardness testing were used as the principal analytical tools for the failure investigation. The findings showed that the failure mechanism is rotating bending fatigue under low stress–high stress concentration conditions. The primarily inadequate component design resulted in high stress concentration factors interacted with the soft annealed material used for the component and caused fatigue under the presence of fluctuating stresses.     

Investigation of the effects of plasma arc parameters on the structure variation of AISI 304 and St 52 steels

In this study, AISI 304 stainless steel and St 52 carbon steel have been cut by plasma arc and the variations of structural specifications occurred after cutting has been investigated. According to the experimental results, it has been seen that burning of particulars and distribution amount were increased when the cutting was performed using the speeds which are upper or lower limits of the ideal cutting speeds proposed by the manufacturer of the machine tool. Moreover, it was determined that the hardness from the outer surface to the core decreased, while the hardness near to the outer surface which affected by the high temperature occurred during cutting increased.     

Investigation of wear behaviours of Al matrix composites reinforced with different B4C rate produced by powder metallurgy method

In this study, the effects of wear behaviours of Al matrix composites reinforced with different B₄C rate produced by powder metallurgy method were investigated. Al and B₄C powders with purity of 99.9% and sizes of 25–44 µm were prepared as pure Al, 4% B₄C/Al, 8% B₄C/Al, 12% B₄C/Al and 16% B₄C/Al. After these prepared mixtures were pressed under 350 MPa, they were sintered for 90 min at 580 °C in atmospheric environment. Microhardness and wear tests of the produced samples were carried out. Wear experiments of these composites were performed with specially manufactured test equipment at different application loads (5 N, 10 N and 15 N), different sliding distances (250 m, 500 m, 750 m and 1000 m) and a constant speed of 0.46 m/s. In addition, optical microscope, SEM, EDS analyses were used to determine the microstructural changes in the worn and unworn surface of the manufactured composite materials. The results of experimental studies show that the increasing the B₄C reinforced rate in composites with Al matrix has led to increase of the hardness and to reduce of the wear loss.     

Influence of remelting on microstructure,hardness and corrosion behaviour of AlCoCrFeNiTi high entropy alloy

Abstract

High entropy alloys are a newly developed class of alloys, which tend to form a single solid solution or a mixture of solid solutions with simple crystal structures. These alloys possess excellent mechanical properties, thermal stability and corrosion resistance. In the present paper, an AlCoCrFeNiTi high entropy alloy was obtained by induction melting, and the influence of the remelting process on the mechanical and corrosion resistance characteristics of the alloy was investigated. Thus, optical and scanning electron microscopy revealed less phase segregation and a fine dendritic structure for the remelted alloy, while corrosion tests indicated that present alloy, in remelted state, has better corrosion resistance than as cast alloy and stainless steel. The Vickers microhardness measurements demonstrated an improvement of the alloy microhardness by remelting process due to the decrease in phase segregation and the increase in dendrite refinement level.