Ferrite Stringers Research Articles

Cold Metal Transfer Welding of Ferritic and Austenitic Stainless Steel: Microstructural, Mechanical, and Electrochemical Studies

In the present study, cold metal transfer arc welding was employed to weld the 304L austenitic stainless steel (ASS) and Ti-stabilized 439 ferritic stainless steel (FSS) using a 309L filler electrode. Dissimilar joints were prepared using low heat input (HI; W1 ~ 247 J/mm) and high HI (W2 ~ 282 J/mm). The solidification mode for both weldments were the ferritic-austenitic mode and the weld zone (WZ) regions of both the weldments consists of columnar austenites, lathy and skeletal ferrite phases. The interfaces between WZ and ASS base metal showed the unmixed zone, whereas a conventional heat-affected zone (HAZ) was formed between the WZ and FSS base metals. The formation of ferrite stringers were observed in the unmixed zone, whereas peppery features of chromium-rich carbides were observed in HAZ. Moreover, electron backscattered diffraction technique was used to distinguish the microstructural differences between W1 and W2 weldments. Increase in the HIs resulted in decreased ferrite fraction in WZ as well as decrease in the mechanical strength of the joints. The W1 weldment depicted higher values of average micro-hardness (WZ ≈ 334.32 HV) than W2 (WZ ≈ 310.92 HV)) weldment. The electrochemical behaviour of the weldments was analysed for both the base metals and WZ of weldments. The higher degree of sensitization (DOS ~ 9.24%) of W1-WZ showed lower intergranular corrosion resistance than W2-WZ (DOS ~ 7.77%), however, the opposite trend was observed for impedance and pitting resistance.

Investigation on microstructural and mechanical integrity of GTAW dissimilar welded joint of IN 718/ASS 304L using Ni-based filler IN 625 using EBSD and DHD techniques

The current study investigates the relationship between the microstructural and structural characteristics of dissimilar gas tungsten arc welding (GTAW) between Inconel 718 (IN 718) and austenitic stainless steel 304L (ASS 304L) using Inconel 625 (IN 625) as a filler material. The microstructural characterization of the dissimilar weld includes the application of an optical microscope (OM), field emission scanning electron microscopy (FE-SEM) including energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The optical and FESEM analysis of the base metals (BM) indicates the austenitic nature of the IN 718 BM matrix, which incorporates strengthening precipitates γ′ and γ'' distributed throughout the Nickel matrix. In contrast, the ASS 304L BM exhibits an austenitic microstructure along with twins. Moreover, IN 625 weld metal exhibits austenitic microstructure in the form of cellular, columnar, and equiaxed dendrites. Elemental segregation demonstrates the existence of Nb, and Mo-rich carbides, including the laves phases at the interdendritic regions of the IN 625 weld zone and IN 718 Heat-Affected Zone (HAZ). Ferrite stringers were noticed in the HAZ of ASS 304L. EBSD analysis shows the improved texture of the weld zone exhibiting fine equiaxed recrystallized microstructures. EBSD/Kernel average misorientation (KAM) micrographs reveal the existence of high strain at the weld metal grain boundaries. Vickers microhardness assessment of the dissimilar weld demonstrates the high hardness of 257.5 HV for the IN 625 weld zone because of the occurred NbC, laves phases, and Mo-rich carbides. Room temperature tensile characterization shows the failure of tensile specimens from ASS 304L BM reflecting the high strength of the IN 625 weld metal (662 MPa). Tensile tests at high temperatures were performed at 550 °C, 600 °C, and 650 °C. High-temperature tensile result exhibits a higher tensile strength of IN 625 weld zone than the ASS 304L BM, as the tensile specimens fails from 304L BM. The specimen exposed at 550 °C exhibits the highest tensile strength of 388 MPa. The Charpy impact toughness test was conducted, with notches at the HAZ of ASS 304L, HAZ of IN 718, and the weld center being evaluated. The toughness value of the weld metal decreases (99 J) due to occurrence of the secondary precipitates. To analyze residual stress, the deep hole drilling (DHD) technique was used, which revealed that the peak residual stress (230 MPa) is induced at 6 mm from the weld surface and falls into the tensile stress category.

Characterization of microstructure, mechanical and corrosion response in AISI 304L and Ti-stabilized 439 stainless steels weld joints

The gas tungsten arc welding (GTAW) was employed to weld dissimilar materials of 304L austenitic stainless steel (ASS) and Ti-stabilized 439 ferritic stainless steel (FSS). Dissimilar joints were produced using 309L ASS filler electrode with low and high heat inputs (HIs). Austenite and ferrite phases were observed in the weld zone (WZ) of both weldments with varying volumetric fractions. The results showed that the ferrite fraction in WZ decreased with increase in HI from 37.2 % (low HI) to 32.39 % (high HI). Formation of ferrite stringers was observed in the unmixed zone of ASS side, whereas chromium-rich carbides were observed in heat affected zone (HAZ) of FSS side. Higher fraction of carbides was observed for the high HI weldment. Mechanical properties of the low HI weldment were superior to high HI weldment due to higher ferrite fraction. Significant differences in microhardness values were observed for the WZ and HAZ between the two HIs weldments. The electrochemical behaviour of the both the weldments were analysed through the various techniques. The results showed that intergranular corrosion resistance was lower for low HI-WZ than high HI-WZ region, however, opposite trend was observed for impedance and pitting resistance.

A Scale-up Study on Chemical Segregation and the Effects on Tensile Properties in Two Medium Mn Steel Castings

Two ingots weighing 400 g and 5 kg with nominal compositions of Fe–8Mn–4Al–2Si–0.5C–0.07V–0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, while the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenized at 1250 ^{circ} C for various times to study the degree of chemical homogeneity and delta -ferrite dissolution. Within 2 hours, the Mn segregation range (max–min) decreased from 8.0 to 1.7 wt pct in the 400 g ingot and from 6.2 to 1.5 wt pct in the 5 kg ingot. Some delta -ferrite also remained untransformed after 2 hours in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modeling was carried out, and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed delta -ferrite in the 5 kg ingot turned into coarse delta -ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed >900 MPa yield strength, >1100 MPa tensile strength, and >40 pct elongation with <10 pct difference in strength and no change in ductility when compared to a fully homogenized sample.

Study on Flux assisted-Tungsten inert gas welding of bimetallic P92 martensitic steel-304H austenitic stainless steel using SiO2–TiO2 binary flux

In present work, flux assisted-tungsten inert gas (FA-TIG) welding of bimetallic P92 martensitic steel-304H austenitic stainless steel (ASS) is carried out using SiO2–TiO2 binary flux. Seven binary fluxes were prepared by varying the proportions of flux powders (SiO2 and TiO2) and the effect of flux composition on weld bead cross-section was studied. The experimental results indicated that binary flux (flux B) with composition 90% SiO2 + 10% TiO2 yields the most pronounced effect on weld bead cross-section with 289.9% improvement in the depth of penetration (DoP) and 58.48% reduction in weld width (WW) in comparison to TIG (no flux) weld joint. To understand the potential mechanisms (arc constriction and reversal of Marangoni's convection) account for improved weld bead cross-section, the efficacy of flux compositions on welding arc/pool behavior is discussed in detail. The addition of a little quantity of TiO2 (10%) in SiO2 (90%) flux helped in achieving the narrowest arc (arc diameter: 5.40 mm), the maximum increase in arc voltage (ΔV = 2.14 V) and an increase in nascent oxygen amount (151 ppm) in the weld pool as compared to TIG (no flux) welding, which confirmed the occurrence of both arc constriction and reversal of Marangoni's convection. The weld joint developed using optimized binary flux (flux B) possessed untempered lath martensite, prior austenite grain boundaries, ferrite stringers and equiaxed austenite grains in the various zones of the weld joint. The average hardness of the weld zone was obtained as 415.9 HV. The developed weld joint qualified the tensile test with ultimate tensile strength (UTS) 680.9 MPa, yield strength (YS) 340.5 MPa and elongation 31.4%.

Leak failure at the TP316L welds of a water pipe caused by microbiologically influenced corrosion

Water leaking through a pinhole from an ASTM A312-TP316L stainless steel pipe was investigated. The pipes had been used for transferring a large amount of potable water. Multiple leaks were found at the circumferential welds of the pipe after a short term of service. An analysis using an optical microscope, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) was conducted to explore the root cause of the failure.The results demonstrated that the perforation of the pipe was due to localized corrosion initiated from the inside at the many small pits penetrating to the outside of the pipe. Corrosion pits were located near the heat-affected zone (HAZ), which is the area most susceptible to a corrosion attack. The microbiologically influenced corrosion (MIC) was identified as the primary cause of perforation failure. Corrosion attacks occurred predominantly at the microstructure of ferrite stringers. Spheroidal corpuscles were observed with a high concentration of sulfur, which indicated that sulfate-reducing bacteria (SRB) were creating a biofilm, accelerating the MIC corrosion process. Slightly concave surfaces near the weld of the pipe exhibited evidence of bacterial attack or residue of microorganisms, where the leak occurred.

A-TIG welding of dissimilar P92 steel and 304H austenitic stainless steel: Mechanisms, microstructure and mechanical properties

The present study focuses on the dissimilar joining of P92 steel and 304H austenitic stainless steel plates of thickness 8 mm using A-TIG welding with aim to select a proper flux for through thickness penetration. The effect of four different oxide fluxes named: Cr2O3, MoO3, SiO2 and TiO2 on weld bead geometry was studied. A detailed investigation on possible mechanism for increase in depth of penetration has been carried out. Out of four oxide fluxes, TiO2 provided through thickness penetration. Further, the integrity of weld joint developed using TiO2 flux was assessed in light of microscopy, hardness test, tensile test and impact toughness test. Microstructure study of the weld joint showed the formation of untempered martensite, polygonal austenite grains and ferrite stringers in different zones of weld joint. In tensile testing, the specimens were failed from 304H austenitic stainless steel base metal. The tensile properties were found as tensile strength 688.6 MPa, yield strength 562.2 MPa and elongation 37.4%. The V-notch Charpy impact toughness of the weld joint (30 J) was quite lower than the base metals due to the formation of untempered martensitic structure.

Sensitization and Stress Corrosion Crack Response of Dual Certified Type 304/304L Stainless Steel

Sensitization has previously been implicated in the stress corrosion cracking (SCC) susceptibility of carbon-containing Type 304 stainless steel (SS) in oxygenated water. Thus, Type 304L SS, with low carbon content, is expected to be resistant to sensitization and therefore SCC susceptibility. The current work evaluates the SCC response of two heats of dual certified Type 304/304L SS that exhibit different microchemical characteristics. One heat contains elevated boron content, while the other heat has delta ferrite stringers in the matrix. It is shown that both heats can be susceptible to SCC because of their unique microchemical features under certain combinations of heat treatment, water chemistry, and deformation level, while under other conditions they remain relatively resistant to SCC.

Development and microstructure characterization of single and duplex nitriding of UNS S31803 duplex stainless steel

In this work, the development of a duplex nitriding (DN) surface treatment combining High Temperature Gas Nitriding (HTGN) and Low Temperature Plasma Nitriding (LTPN) is reported. The microstructure and hardness variation of the duplex treated steel is compared with the properties obtained during single HTGN and single LTPN of UNS S31803 stainless steel. Single LTPN of UNS S31803 was carried out in an Active Screen Plasma Nitriding reactor, at 400°C for 20h, in a 75% N2+25% H2 atmosphere. Single HTGN of UNS S31803 was carried out at 1200°C, under a 0.1MPa high purity N2 gas atmosphere, during 8h. The developed duplex nitriding (DN) surface treatment consists of a combination of both, HTGN and LTPN treatments, carried out in the same conditions described above.The microstructure of the as received material was composed by ferrite and austenitic stringers, aligned in the rolling direction. The results showed that LTPN of the UNS S31803 duplex stainless steel promotes the formation of a duplex modulated structure composed by 2.5μm thick, 1510±52HV hard, expanded ferrite (αN) regions, and 3.0μm thick, 1360±81HV hard, expanded austenite (γN) regions on ferrite and austenite grains, respectively. Intense coherent ε-Fe3N nitride precipitation inside expanded ferrite was observed. ε-Fe3N nitrides precipitated with an orientation relationship [111] αN//[120] ε-Fe3N, leading to increased microhardness of the expanded ferrite regions. After the first step of the duplex nitriding treatment (HTGN) a 550μm thick, 330HV hard, nitrogen rich, fully austenitic layer formed at the surface of the specimens, by transformation of ferrite stringers into austenite. The second nitriding step (LTPN) led to the formation of a homogeneous expanded austenite layer, 1227±78HV on top of the thick fully austenitic layer, formed during the first step. The duplex treatment resulted in a more homogeneous, precipitate-free, microstructure and a better transition between the mechanical properties of the hardened outermost layer and the softer substrate.

A Review on the Role of Microstructural Heterogeneities in the Failure of Aerospace Components: A Metallographer’s Perspective

Wrought components show improved mechanical properties under both static and dynamic conditions against their cast counterparts. This is basically attributed to the homogeneous and fine microstructure introduced by the thermomechanical processing. However, thermomechanical processing also leads to mechanical property anisotropy especially when the microstructural heterogeneities (second phase, inclusions) are not randomly distributed and get oriented. This usually occurs due to the nature of the thermomechanical processing adopted to fabricate the products. The microstructural heterogeneities can be both metallic (such as delta ferrite stringers in steels) and non-metallic (such as MnS inclusions in steel) in nature. Physical orientation of grains and the crystallographic orientation due to thermomechanical processing also add to the woes of heterogeneities in these materials. When materials with non-metallic inclusions are subjected to large plastic deformation, such as wire drawing and are used for applications such as springs (which are subjected to dynamic loading), failures can occur in these components due to the propagation of cracks from the incoherent interface between inclusion and matrix. Similarly, when these materials are subjected to secondary fabrication processes such as cold swaging or high strain rate deformation, failures can occur due to the propagation of cracks along the interface of the second phase with the matrix. Therefore, the purpose of the present paper is to highlight the role of microstructural heterogeneities in the failure of components with specific emphasis to aerospace structural materials. Finally, it is concluded that materials subjected to dynamic loading conditions during fabrication or service should be microstructurally clean and second phase, if present should be fragmented and uniformly distributed in the matrix to avoid cracking/failure.

Role of grain boundary ferrite layer in dynamic recrystallization of semi-solid processed type 304L austenitic stainless steel

A semi-solid processed steel with well arranged delta-ferrite layer around the austenite-solid globules is found to show different dynamic recrystallization (DRX) pattern compared to its conventionally processed counterpart with ferrite stringers across the austenite grains. Analysis of experimental results indicates that the presence of delta-ferrite layer around austenite-globules delays the nucleation of new DRX grains. On the other hand, with increase in temperature, the delta-ferrite layer contributes to increase in DRX fraction by restricting the grain growth of the austenite matrix prior to deformation.

Unraveling the Effect of Thermomechanical Treatment on the Dissolution of Delta Ferrite in Austenitic Stainless Steels

Considering the detrimental effects of delta ferrite stringers in austenitic stainless steels and the industrial considerations regarding energy consumption, investigating, and optimizing the kinetics of delta ferrite removal is of vital importance. In the current study, a model alloy prone to the formation of austenite/delta ferrite dual phase microstructure was subjected to thermomechanical treatment using the wedge rolling test aiming to dissolve delta ferrite. The effect of introducing lattice defects and occurrence of dynamic recrystallization (DRX) were investigated. It was revealed that pipe diffusion is responsible for delta ferrite removal during thermomechanical process, whereas when the DRX is dominant, the kinetics of delta ferrite dissolution tends toward that of the static homogenization treatment for delta ferrite removal that is based on the lattice diffusion of Cr and Ni in austenite. It was concluded that the optimum condition for dissolution of delta ferrite can be defined by the highest rolling temperature and strain in which DRX is not pronounced.

Metallurgical Analysis of a Failed 17-4 PH Stainless Steel Pyro Bolt Used in Launch Vehicle Separation Systems

Abstract Precipitation hardenable stainless steels are widely used in aerospace applications because of their excellent combination of strength and corrosion resistance. In view of this advantage, Fe-17Cr-4Ni (17-4 PH) steel is used in pyro bolts in space ordnance systems for stage separation in a satellite launch vehicle. These systems typically consist of a pyro charge to sever a given material resulting in stage separation, and these materials are exposed to high strain rates. In one such application, although perfect severance took place, secondary cracks were noticed longitudinally, which are not desirable. This steel, in view of its composition, has delta ferrite in a martensitic matrix in the hardened and tempered condition. In wrought products worked unidirectionally, delta ferrite gets oriented along the direction of thermomechanical processing. The effect of delta ferrite orientation on the impact properties of 17-4 PH stainless steel was studied in various heat-treatment conditions with a view to understand the effect of microstructure on impact properties. It was found that the transverse impact properties are severely affected by the presence of highly oriented delta ferrite stringers for any given heat-treatment condition. This has been attributed to the weak delta ferrite–martensite interface, which facilitates easy crack propagation. Finally, it was suggested that the anisotropy in impact properties need to be carefully considered in the design of components for applications that involve high strain rates.

Effect of Continuous and Pulsed Current on the Metallurgical and Mechanical Properties of Gas Tungsten Arc Welded AISI 4340 Aeronautical and AISI 304 L Austenitic Stainless Steel Dissimilar Joints

In this research work, the weldability of low alloyed AISI 4340 aeronautical steel and AISI 304L austenitic stainless steel joined by continuous current (CC) and pulsed current (PC) gas tungsten arc welding (GTAW) techniques, using ER309L and ERNiCr-3 filler metals was investigated. The main focus of the study involves the investigation on the effect of continuous and pulsed current mode of GTA welding process on the metallurgical and mechanical properties of these dissimilar weldments. Microstructure studies revealed the formation of different zones across the weldments, vis-a-vis martensite at the HAZ of AISI 4340, vermicular δ - ferrite /ferrite stringers at the HAZ of AISI 304L, pearlite colonies at the parent metal of AISI 4340 and equi-axed cellular and/or columnar dendrites at the weld zone. Tensile results showed that current pulsing accrued better tensile properties. The structure - property relationships of these weldments were established based on the current modes employed by utilizing combined techniques of optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

Microstructural Characterization and Corrosion Behavior of Activated Flux Gas Tungsten Arc-Welded and Multipass Gas Tungsten Arc-Welded Stainless Steel Weld Joints in Nitric Acid

AISI Type 304L (UNS S30403) austenitic stainless steels are widely used in spent nuclear fuel reprocessing plants, and welding is an indispensable tool used for joining these materials. In the present study, manual gas tungsten arc-welded (M-GTAW) and activated gas tungsten arc-welded (A-GTAW) weldments of Type 304L stainless steel were prepared to examine the microstructural and corrosion behavior of the weldments. A total of 6 passes were required to complete the 6 mm thick sample welding, and 16 passes were required for 12 mm thick sample welding using the M-GTAW process, compared to single-pass A-GTAW welding. Characterization of weld joints was done by radiography, optical microscopy, microhardness tester, a feritscope, and scanning electron microscopy (SEM). The optical microstructure of the fusion zone of weld joints showed delta ferrite in various morphologies. The presence of delta ferrite stringers were observed in the weld joints, extending from the weld metal to the base metal. The corrosion rate results showed that the M-GTAW sample showed only a marginal increase in the corrosion resistance when compared to those welded by the single-pass A-GTAW process. SEM examination revealed the morphology of attack in the base metal was predominantly intergranular while in the weld metal it was interdendritic. The SEM micrograph also showed preferential attack of the delta ferrite stringers.

TIG AISI-316 welds using an inert gas welding chamber and different filler metals: Changes in mechanical properties and microstructure

This report analyses the influence of the use of an inert gas welding chamber with a totally inert atmosphere on the microstructure and mechanical properties of austenitic AISI 316L stainless steel TIG welds, using AISI ER316L, AISI 308L and Inconel 625 as filler metals. When compared with the typical TIG process, the use of the inert gas chamber induced changes in the microstructure, mainly an increase in the presence of vermicular ferrite and ferrite stringers, what resulted in higher yield strengths and lower values of hardness. Its effect on other characteristics of the joins, such as tensile strength, depended on the filler metal. The best combination of mechanical characteristics was obtained when welding in the inert gas chamber using Inconel 625 as filler metal.

Effect of hydrogen charging on fracture behaviour of 304L stainless steel

AbstractSlow tensile straining of a series of specimens of 304L stainless steel after thermally charging with hydrogen at 31·0 MPa and 350°C resulted in a ductility loss compared with uncharged specimens. The susceptibility to embrittlement was shown to be dependent on the formation of martensite during deformation and, hence, the stability (and composition) of the austenite, but the interface between the austenite and any ferrite stringers acted as a nucleation site for cracking and as a weak propagation path for fracture.MST/1088

Effect of hydrogen charging on fracture behaviour of 304L stainless steel

Slow tensile straining of a series of specimens of 304L stainless steel after thermally charging with hydrogen at 31·0 MPa and 350°C resulted in a ductility loss compared with uncharged specimens. The susceptibility to embrittlement was shown to be dependent on the formation of martensite during deformation and, hence, the stability (and composition) of the austenite, but the interface between the austenite and any ferrite stringers acted as a nucleation site for cracking and as a weak propagation path for fracture.MST/1088

Influence of specimen size and microstructure on the fracture toughness of a martensitic stainless steel

The fracture toughness of alloy HT-9, † † HT-9 is a product of the Sandvikens Jernverks AB, Sweden. a martensitic stainless steel under consideration for fast reactor and fusion reactor applications, was determined from circular compact tension specimens using the multi-specimen R-curve method. Specimens with thicknesses of 11.94, 7.62 and 2.54mm and widths of 23.88 and 11.94 mm were tested to investigate the effects of specimen size on fracture toughness. The test results obtained from all specimens are in good agreement and thickness requirements for a valid J 1 c test are satisfied. The experiment indicates that small specimens of HT-9 may be used for post-irradiation fracture toughness testing. Fractographic examination of the fracture surfaces reveals that fracture in HT-9 is significantly influenced by delta ferrite stringers present in the material. The fracture surface examination and crack opening displacement measurements for specimens tested at various temperatures are consistent with the temperature dependence of the J 1 c results.