Vani Shankar

Microstructural features of dissimilar welds between 316LN austenitic stainless steel and alloy 800

Abstract For joining type 316LN austenitic stainless steel to modified 9Cr–1Mo steel for power plant application, a trimetallic configuration using an insert piece (such as alloy 800) of intermediate thermal coefficient of expansion (CTE) has been sometimes suggested for bridging the wide gap in CTE between the two steels. Two joints are thus involved and this paper is concerned with the weld between 316LN and alloy 800. These welds were produced using three types of filler materials: austenitic stainless steels corresponding to 316, 16Cr–8Ni–2Mo, and the nickel-base Inconel 182 1 . The weld fusion zones and the interfaces with the base materials were characterised in detail using light and transmission electron microscopy. The 316 and Inconel 182 weld metals solidified dendritically, while the 16–8–2 (16%Cr–8%Ni–2%Mo) weld metal showed a predominantly cellular substructure. The Inconel weld metal contained a large number of inclusions when deposited from flux-coated electrodes, but was relatively inclusion-free under inert gas-shielded welding. Long-term elevated-temperature aging of the weld metals resulted in embrittling sigma phase precipitation in the austenitic stainless steel weld metals, but the nickel-base welds showed no visible precipitation, demonstrating their superior metallurgical stability for high-temperature service.

Solidification cracking in austenitic stainless steel welds

Solidification cracking is a significant problem during the welding of austenitic stainless steels, particularly in fully austenitic and stabilized compositions. Hot cracking in stainless steel welds is caused by low-melting eutectics containing impurities such as S, P and alloy elements such as Ti, Nb. The WRC-92 diagram can be used as a general guide to maintain a desirable solidification mode during welding. Nitrogen has complex effects on weld-metal microstructure and cracking. In stabilized stainless steels, Ti and Nb react with S, N and C to form low-melting eutectics. Nitrogen picked up during welding significantly enhances cracking, which is reduced by minimizing the ratio of Ti or Nb to that of C and N present. The metallurgical propensity to solidification cracking is determined by elemental segregation, which manifests itself as a brittleness temperature range or BTR, that can be determined using the varestraint test. Total crack length (TCL), used extensively in hot cracking assessment, exhibits greater variability due to extraneous factors as compared to BTR. In austenitic stainless steels, segregation plays an overwhelming role in determining cracking susceptibility.

A comparative evaluation of welding consumables for dissimilar welds between 316LN austenitic stainless steel and Alloy 800

Abstract Transition joints in power plants between ferritic steels and austenitic stainless steels suffer from a mismatch in coefficients of thermal expansion (CTE) and the migration of carbon during service from the ferritic to the austenitic steel. To overcome these, nickel-based consumables are commonly used. The use of a trimetallic combination with an insert piece of intermediate CTE provides for a more effective lowering of thermal stresses. The current work envisages a trimetallic joint involving modified 9Cr–1Mo steel and 316LN austenitic stainless steel as the base materials and Alloy 800 as the intermediate piece. Of the two joints involved, this paper describes the choice of welding consumables for the joint between Alloy 800 and 316LN. Four consumables were examined: 316, 16-8-2, Inconel 82 and Inconel 182. The comparative evaluation was based on hot cracking tests and estimation of mechanical properties and coefficient of thermal expansion. While 16-8-2 exhibited highest resistance to solidification cracking, the Inconel filler materials also showed adequate resistance; additionally, the latter were superior from the mechanical property and coefficient of thermal expansion view-points. It is therefore concluded that for the joint between Alloy 800 and 316LN the Inconel filler materials offer the best compromise.

Effect of nitrogen addition on microstructure and fusion zone cracking in type 316L stainless steel weld metals

Abstract Nitrogen is known to have a significant effect on cracking behaviour of austenitic stainless steel during welding, although reports on its effects have often been controversial. A study was therefore undertaken to examine the effect of nitrogen on the weldability of two type 316L weld metals. Weldability was assessed using the longitudinal moving torch Varestraint test. The brittleness temperature range during solidification was calculated from crack length data. Nitrogen was added through the shielding gas to 316L (base N-0.036%) and 316LN (base N-0.073%) to produce weld metal nitrogen contents in the range 0.04–0.19%. In the primary austenitic solidification mode, nitrogen addition had little effect when the P+S levels were relatively low (316LN with 0.031%P, 0.001%S) while cracking increased for higher impurity levels (316L with 0.035%P, 0.012%S). Nitrogen additions also produced significant coarsening of the primary solidification structure. The study indicates that weldability effects of nitrogen may be influenced by the impurity levels, particularly S. The cracking data showed good correlation with the WRC Cr eq /Ni eq ratio.

Correlation of microstructure and mechanical properties with ultrasonic velocity in the Ni-based superalloy Inconel 625

Abstract Inconel 625 tubes are used extensively in ammonia cracker units of heavy-water plants. During service, the alloy is exposed to temperature close to 873 K for a prolonged period (about 60 000 h), leading to substantial decrease in ductility and toughness of the alloy due to heavy intragranular and intergranular precipitation. Service-exposed Inconel 625 material (873 K for about 60 000 h) was given post-service ageing treatments at different temperatures (923, 1023 and 1123K) up to 500 h. These heat treatments altered the microstructure, which in turn had an influence upon both the tensile properties and the ultrasonic velocity. The alloy that had seen service was solution annealed at 1423 K for 0.5 h followed by ageing at different temperatures (923 and 1123 K) and the influence of these heat treatments on changes in microstructures and in turn their effect on room-temperature tensile properties and ultrasonic velocity have been studied. The present study aims at establishing the correlation between room-temperature tensile properties and ultrasonic velocity with the microstructural changes that occurred during ageing treatments in Ni-based superalloy Inconel 625. For the first time, the present authors have demonstrated the influence of various precipitates, such as intermetallic phases γ″, Ni2(Cr, Mo) and δ, and grain-boundary carbides, on the correlation of yield strength and ultrasonic velocity.

Criteria for hot cracking evaluation in austenitic stainless steel welds using longitudinal varestraint and transvarestraint tests

Abstract The longitudinal varestraint test (LVT) and transvarestraint test (TVT) are widely used for assessment of weld metal cracking susceptibility. The TVT is preferred over the LVT for study of weld metal cracking. However, few reports exist that discuss the relative merits of the two tests for evaluating cracking susceptibility. This investigation was carried out to compare weldability assessments using the two tests and the relevant criteria for weldability evaluation. Several stainless steels solidifying in the austenitic and ferritic solidification modes were tested. The study shows that the LVT can be used for evaluation of fusion zone cracking through a maximum cracking distance criterion. This parameter correlated well with the maximum crack length in the TVT, traditionally used to derive the brittleness temperature range (BTR). The study further indicates that the total crack length can be related to the BTR by considering the area density of cracking.

The effect of diffusion barrier formation on the kinetics of aluminizing in inconel-718

Aluminizing of nickel alloy 718 was studied in order to reveal the effect of combined alloy additions on aluminide growth kinetics, as opposed to pure metal substrates. The low activity pack process was used and treatment was carried out at 1273 K using ammonium fluoride activator and Ni-50 wt% Al powder as an aluminium source for treatment times of 1, 2, 4 and 8 h. The aluminide coatings varied between 40 and 110 Μm in thickness. The microstructures consisted of a NiAl phase with a fine grain size containing small secondary-phase particles at the grain boundaries. The interface between the coating and the substrate was lined by a lamellar layer exhibiting a two-phase structure, which was enriched in chromium and niobium in addition to containing iron and nickel. Weight gain measurements indicated parabolic growth up to 2 h, beyond which the growth rate slowed down. Microstructures and composition profiles revealed that the interlayer, which was enriched in elements insoluble in NiAl, posed a barrier to interdiffusion of the reacting species and slowed down the growth kinetics of the aluminide.

Effect of Application of Short and Long Holds on Fatigue Life of Modified 9Cr-1Mo Steel Weld Joint

Modified 9Cr-1Mo steel is a heat-treatable steel and hence the microstructure is temperature sensitive. During welding, the weld joint (WJ) is exposed to various temperatures resulting in a complex heterogeneous microstructure across the weld joint, such as the weld metal, heat-affected zone (HAZ) (consisting of coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ), and the unaffected base metal of varying mechanical properties. The overall creep–fatigue interaction (CFI) response of the WJ is hence due to a complex interplay between various factors such as surface oxides and stress relaxation (SR) occurring in each microstructural zone. It has been demonstrated that SR occurring during application of hold in a CFI cycle is an important parameter that controls fatigue life. Creep–fatigue damage in a cavitation-resistant material such as modified 9Cr-1Mo steel base metal is accommodated in the form of microstructural degradation. However, due to the complex heterogeneous microstructure across the weld joint, SR will be different in different microstructural zones. Hence, the damage is accommodated in the form of preferential coarsening of the substructure, cavity formation around the coarsened carbides, and new surface formation such as cracks in the soft heat-affected zone.

Evaluation of Hot Cracking Susceptibility of Some Austenitic Stainless Steels and a Nickel-Base Alloy

Abstractfor the Prototype Fast Breeder Reactor (PFBR), a modified version of 316L stainless steel, designated as 316L(N), has been chosen as the major structural material. In order to reduce the risk of sensitisation, the carbon content has been reduced to less than 0.03 wt-%, and to compensate for the loss in strength due to the reduced carbon content the nitrogen content has been specified to be about 0.08 wt-%. For fuel clad and wrapper applications, a radiation-resistant variation of 316 stainless steel containing titanium about 6 times the carbon content, named Alloy D9, has been chosen. Weld metal and heat-affected zone (HAZ) cracking of austenitic stainless steels Alloy D9 and 316L(N) were investigated. Specifically, the role of titanium in Alloy D9 and nitrogen in 316L(N), along with the impurity elements, were studied. In Alloy D9, cracking increased with Ti/C ratio, but a significant contribution to cracking came from the nitrogen of about 200 ppm picked up during welding even when using high purity argon shielding gas. Titanium to carbon (Ti/C) ratio of about 4 was found to show least susceptibility to solidification as well as HAZ cracking. In modified 316 weld metals with 3–7 FN, nitrogen in the range 0.06–0.12 % had no detrimental effect on weldability. Weldability of Inconel 718 base material was also investigated. From hot cracking considerations, ENiCrFe-3 consumable was found more suitable to weld Inconel 718 than consumable of matching composition. Weldability was tested in various geometrical configurations such as T-, butt- and rod-to-strip in similar as well as dissimilar combination with 9Cr-1Mo steel using ENiCrFe-3 consumable. The studies showed the need for careful joint preparation and use of techniques to enhance weld penetration for minimising defects. This paper discusses the weldability problems associated with these austenitic stainless steels chosen for use in the construction of PFBR. Various criteria in use for weldability evaluation as per codes in relation to the present data on stainless steels and nickel-base alloys are also discussed. The importance of hot cracking evaluations in determining the fabrication weldability of these austenitic stainless steels is also discussed in detail.

Impact toughness of 316 stainless steel weld metal on elevated temperatures aging

Abstract Shielded metal arc welding electrodes of a modified E316-15 austenitic stainless steel, for service at 673–823 K with delta ferrite in the range of 3–7 ferrite number, have been developed indigenously for welding of 316L(N) stainless steel structural materials for the Indian Prototype Fast Breeder Reactor. Delta ferrite content in weld metals for high temperature service is restricted for limiting the formation of embrittling secondary phases during service. To study the effect of high temperature exposure on microstructure and mechanical properties, the 316 weld metal was aged at three different temperatures of 923, 973 and 1023 K, for various durations up to 500 h. The activation energy for the transformation of delta ferrite has been estimated to analyse the mechanism associated with the micro structural changes that led to the deterioration in toughness on elevated temperature aging of this weld metal.