H. Shaikh

Stress corrosion crack growth studies on nitrogen added AISI type 316 stainless steel and its weld metal in boiling acidified sodium chloride solution using the fracture mechanics approach

Compact tension specimens of nitrogen-added AISI type 316 austenitic stainless steel and its weld metal were subjected to stress corrosion cracking (SCC) testing in a boiling solution containing 5 M sodium chloride + 0.15 M sodium sulphate + 2.5 ml/l hydrochloric acid solution using the constant extension rate testing (CERT) technique. The extension rate of testing was 10 microns per hour. The threshold values of stress intensity factor (KISCC) and J-integral (JISCC) were taken as those values of KI and JI at which about 25 microns of SCC crack growth was observed. These threshold values were about four times higher and plateau crack growth rates (PCGR) were nearly one order of magnitude lower for the base metal vis-a-vis the weld metal. Fractographic observations indicated failure by transgranular SCC (TGSCC) of austenite in both the base and weld metal. No stress-assisted dissolution of delta-ferrite or its interface with austenite, was observed. Bruchmechanische Untersuchung der Spannungsrisskorrosion an Grund- und Schweisnahtwerkstoff des stickstoffhaltigen Stahls AISI 316 LN in siedender Natriumchloridlosung CT-Proben von Grund- und Schweisnahtwerkstoff des stickstoffhaltigen Stahles AISI 316 LN wurden Spannungsrisskorrosionstests in siedender chloridhaltiger Losung (5 M Natriumchlorid/0,15 M Natriumsulfat/0,03 M Salzsaure) unterzogen. Die Tests erfolgten bei konstanter Dehnrate (CERT-Test) von 10 μm/h. Als Schwellwerte der Initiierung von Spannungsrisskorrosion KISCC und JISCC wurden die Werte des Spannungsintensitatsfaktors KI und des J-Integrals JI ermittelt, bei denen ein Risswachstum von 25 μm auftrat. Dabei wies der Grundwerkstoff 4-fach hohere Schwellwerte KISCC und JISCC auf als der Schweisnahtwerkstoff Auch die Risswachstumsraten im Plateaubereich der Risswachstumsrate-Spannungsintensitatskurven waren am Grundwerkstoff um eine Grosenordnung geringer als am Schweisnahtwerkstoff. Die fraktographischen Untersuchungen zeigten an beiden Materialien Schadigung durch transkristalline Spannungsrisskorrosion. Eine spannungsgeforderte Auflosung von Deltaferrit oder der Deltaferrit/Austenit-Grenzflachen konnte dagegen nicht beobachtet werden.

Failure analysis of a T-joint of AISI type 316 L stainless steel

Abstract The flow of sodium through the various circuits of a PFBR is controlled by centrifugal pumps. T-joints, made of type 316 LN stainless steel, will be used as pipe fittings to connect sodium pumps in the secondary side of the PFBR. However, two such T-joints received from the manufacturer were found to have developed fine cracks on the surface, which were seen after the pickling and passivation operation was carried out at the users site. Visual, metallographic and fractographic examinations suggested that the failure occurred due to fatigue, which initiated because of surface roughening caused by the fabrication process. Initial large grain size of the material caused surface roughening to occur during fabrication. The rough surface led to initiation of fatigue cracks during fabrication, which was not carried out in one stretch as recommended by the user. Absence of corrosion product on the surface suggested no role of corrosion in the failure. The high hardness of the material indicated non-compliance with the users specifications by the manufacturer vis-a-vis post-fabrication annealing. To avoid future failures, it was recommended that proper selection of material with respect to starting grain size should be made; qualified fabrication procedures should be adopted and followed so as to avoid cyclic loading on the component during fabrication; and appropriate solution annealing at 1323 K should be carried out to relieve cold work.

On the microstructure-polarization behavior correlation of a 9Cr-1Mo steel weld joint

The use of 9Cr-1Mo ferritic steel necessitates its fabrication by the process of welding. The heat-affected zone (HAZ) of 9Cr-1Mo ferritic steel is a combination of many microstructures. In the present study, the corrosion properties of the base metal, weld metal, and the various regions of the HAZ are assessed with respect to their microstructures. The various microstructures in the HAZ were simulated by heat treatment of the normalized and tempered base metal at 1463, 1200, and 1138 K for 5 min followed by oil quenching. The microstructure of the base metal in the normalized and tempered condition revealed martensite laths with M23C6 carbides at lath boundaries, and uniform dispersion of fine, acicular M2C. The weld metal showed predominantly martensitic structure with dispersion of carbides. Simulation of the microstructures of the HAZ by heat treatment resulted in the following microstructures: coarse-grained martensite of 75 µm size at 1463 K, fine-grained martensite at 1200 K, and martensite + proeutectoid α-ferrite at 1138 K. In all cases, carbide precipitation was observed in the martensitic matrix. Microhardness measurements of HAZ-simulated base metal showed increasing hardness with increasing heat treatment temperature. The hardness values obtained corresponded very well with the regions of the actual HAZ in the weld joint. Electrochemical polarization studies were carried out on the base metal, weld metal, weldment (base metal + weld metal + HAZ), and the simulated HAZ structures in 0.5 M sulfuric acid solution. Critical current densities (icrit1 and icrit2), passive current densities (ipass and isec-pass), and transpassive potential (Etp) were the parameters considered for evaluating the corrosion resistance. The HAZ structures simulated at 1463 and 1200 K, corresponding to coarse- and fine-grained martensitic regions of an actual HAZ, had corrosion properties as good as the normalized and tempered base metal. Of the various simulated HAZ structures, the intercritical region, which was simulated at 1138 K, possessed the worst corrosion resistance. The weld metal possessed the worst corrosion resistance of the various microstructural regions in the weld joint. The weldment adopted the degraded corrosion properties of the weld metal.

Failure analysis of an AM 350 steel bellows

Abstract The failure of an AM 350 steel bellows, which was to be used in the control rod drive mechanism (CRDM) of the fast breeder test reactor (FBTR), was noticed during helium leak testing. The leak test was carried out before performance testing in a test rig. Visual examination of the leak area did not indicate any obvious defect. Stereo microscopy and optical microscopy indicated the presence of pits. A few of these pits had propagated through the thickness. EDAX of the corrosion products revealed the presence of chlorides. The exposure of the bellows to a marine atmosphere during a storage period of 13 years was suspected to have caused the pitting.

Effect of Heat Input on the Stress Corrosion Cracking Behavior of Weld Metal of Nitrogen-Added AISI Type 316 Stainless Steel

Abstract Round tensile specimens of AISI Type 316N (UNS S31651) stainless steel weld metal, made by manual metal arc welding (MMAW) process using heat inputs ranging from 3.07 kJ/cm to 7.41 kJ/cm, were subjected to stress corrosion cracking (SCC) tests in boiling acidified sodium chloride (NaCl) solution (initial stress level = 250 MPa) and in boiling 45% magnesium chloride (MgCl2) solution (initial stress level = 120 MPa), using the constant load technique. In boiling acidified NaCl tests, the open-circuit potential (vs saturated calomel electrode [SCE]) was monitored with respect to time to determine the critical cracking potential (CCP) at the time-of-failure. In boiling acidified NaCl solution, the SCC time-to-failure (tf) increased while the CCP decreased with increasing heat input. In boiling 45% MgCl2 solution, no significant change in tf was observed. The tf in acidified NaCl solution was far greater than that in 45% MgCl2 solution. Failure occurred by a combination of transgranular stress corrosi...

Corrosion failures of AISI type 304 stainless steel in a fertiliser plant

Abstract A urea plant, operating on ammonia and carbon dioxide (CO 2 ) gases, had to be shutdown due to corrosion in the intercooler and aftercooler of its CO 2 gas cleaning circuit. Extensive general corrosion of AISI type 304 stainless steel parts, such as sealing strips, fins, demisters and the shell, of these two components which were in contact with the duplex stainless steel tubes, caused the shutdown of the fertiliser plant within 6 months. Investigations of the corrosion products by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques showed the presence of carbon and ammonia based compounds, thus suggesting the role of ammonia and CO 2 gases, or the product of their reactions, in the corrosion of type 304 stainless steel. Electrochemical polarisation studies showed that duplex stainless steel possessed a more positive open circuit potential and a nobler critical pitting potential than type 304 stainless steel thus confirming that the corrosion of type 304 stainless steel was caused by the galvanic action with the duplex stainless steel heat transfer tubes. Hence, it was recommended that (i) the same material (type 304 stainless steel) be used for all parts of the intercooler and aftercooler to avoid galvanic corrosion, (ii) condense water carried over by CO 2 gas by cooling it to low temperatures immediately after it comes out from the scrubber, (iii) slight modification of the process to add up to 0.8% oxygen in the CO 2 gas before entry into the intercooler, which will help in retaining/formation of an effective passive film on type 304 stainless steel.

Introduction to Austenitic Stainless Steels

Abstract The family of austenitic stainless steels has a wide variety of grades precisely tailored for specific applications such as household and community equipment, transport, food industry, industrial equipment, chemical and power engineering, cryogenics, and building industry. The optimum choice of the grades would depend on service needs and this would require a clear understanding of the metallurgical parameters, which control the microstructure and thus the mechanical properties, formability and corrosion resistance. This chapter, in brief, deals with the physical metallurgy, welding metallurgy, and physical and mechanical properties of austenitic stainless steels. In the physical metallurgy of stainless steels the tendency of alloying elements to form different phases, the transformation of austenite to martensite during cooling or straining, hardening processes and formation of intermetallic phases, have been discussed. The influence of chemical composition, and temperature on the various physical properties of austenitic stainless steel such as coefficient ofexpansion, thermal conductivity and magnetic permeability is highlighted. Variation in mechanical properties, such as tensile, fatigue and creep strengths of austenitic stainless steels with temperature, composition and microstructure has been discussed. The mechanisms to strengthen the austenitic stainless steels by appropriate thermo-mechanical treatments, grain refinement etc. have also been addressed. Austenitic stainless steels lend themselves remarkably to deep drawing and cold rolling, where their work-hardening characteristics enable high strength levels to be attained. Weldability is excellent, and welds, which do not transform to martensite during air-cooling, have mechanical properties similar to base metal.

Corrosion issues in ferrous weldments

The properties of weld metal in a specified environment should be equal to or better than those of the base metal. However, in most cases that is not the case. The main cause for the degradation of steel (austenitic or ferritic) weld joint is the formation of many regions with widely differing microstructures, which respond differently to the environment. The microstructure of a weldment depends on the chemical composition of the filler material, welding process and heat input, which controls the cooling rates in the various regions. These microstructural features deteriorate the general and localised corrosion properties of the weldment. Residual stresses add up to the service stresses and enhance the environment cracking susceptibilities of the weld joint besides increasing susceptibility to other forms of localised corrosion. The influence of microstructural variations in weld metal, sensitisation in heat-affected zone, residual stresses, etc., on general, localised and environmentally assisted cracking behaviour in austenitic stainless steels and ferritic steels are reviewed in this chapter.