Structure-property correlation of submerged-arc and gas-metal-arc weldments in HY-100 steel

Structure-property relationships of two HY-100 steel weldments prepared by submerged arc (SAW) and gas metal arc (GMAW) welding processes using identical heat input (2.2 kJ mm^-1) have been studied. It has been found that submerged arc welded (SAW) HY-100 steel weldments have a lower weld toughness than welds produced by the gas metal arc welding (GMAW) process. Optical, scanning, and transmission electron microscopy were used in conjunction with microhardness traverses to characterize and compare the various microconstituents that are present in the last weld pass of both weldments. TEM examination revealed the presence of coarse upper bainite, B-II bainite, and carbides in a highly dislocated ferrite matrix as well as in ferrite laths in the SAW weldment, while the GMAW weldment exhibited a typical fine low carbon lath martensite, autotempered martensite, and mixed B-II and B-III bainites which occasionally contained small regions of twinned martensite. The measured cooling rate in the SAW was found to be about 40 pct slower than that in GMAW. It was also found in the SAW that the weld metal inclusion number density was about 25 pct greater than that in GMAW. Micro-hardness traverses exhibited significantly lower hardness (about 50 HV) in the SAW weldment compared with GMAW, but the tempered weld metal microhardness in both the weldments was measured about the same, at 250 HV. The ductile-to-brittle transition temperature (DBTT) of both weldments was determined by Charpy impact test. Based on an average energy criterion, the DBTT of the SAW weldment was 323 K (50 °C) higher than that of the GMAW weldment. This difference in fracture resistance is due to the different weld metal microstructures. The different microstructures most probably result from differences in cooling rate subsequent to welding; however, the SAW weld also has a higher inclusion number density which could promote a higher transformation temperature for the austenite.     

Structure-property correlation of submerged-arc and gas-metal-arc weldments in HY-100 steel

Metallurgical and Materials Transactions A - Structure-property relationships of two HY-100 steel weldments prepared by submerged arc (SAW) and gas metal arc (GMAW) welding processes using...     

Structure-property correlation of submerged-arc and gas-metal-arc weldments in HY-100 steel

Structure-property relationships of two HY-100 steel weldments prepared by submerged arc (SAW) and gas metal arc (GMAW) welding processes using identical heat input (2.2 kJ mm^-1) have been studied. It has been found that submerged arc welded (SAW) HY-100 steel weldments have a lower weld toughness than welds produced by the gas metal arc welding (GMAW) process. Optical, scanning, and transmission electron microscopy were used in conjunction with microhardness traverses to characterize and compare the various microconstituents that are present in the last weld pass of both weldments. TEM examination revealed the presence of coarse upper bainite, B-II bainite, and carbides in a highly dislocated ferrite matrix as well as in ferrite laths in the SAW weldment, while the GMAW weldment exhibited a typical fine low carbon lath martensite, autotempered martensite, and mixed B-II and B-III bainites which occasionally contained small regions of twinned martensite. The measured cooling rate in the SAW was found to be about 40 pct slower than that in GMAW. It was also found in the SAW that the weld metal inclusion number density was about 25 pct greater than that in GMAW. Micro-hardness traverses exhibited significantly lower hardness (about 50 HV) in the SAW weldment compared with GMAW, but the tempered weld metal microhardness in both the weldments was measured about the same, at 250 HV. The ductile-to-brittle transition temperature (DBTT) of both weldments was determined by Charpy impact test. Based on an average energy criterion, the DBTT of the SAW weldment was 323 K (50 °C) higher than that of the GMAW weldment. This difference in fracture resistance is due to the different weld metal microstructures. The different microstructures most probably result from differences in cooling rate subsequent to welding; however, the SAW weld also has a higher inclusion number density which could promote a higher transformation temperature for the austenite. Formerly Adjunct Research Professor with the Materials Engineering Group, Naval Postgraduate School Formerly Graduate Student at NPS     

Failure behavior of heat-affected zones within HSLA-100 and HY-100 steel weldments

The deformation and fracture behavior of simulated heat-affected zones (HAZ) within HSLA-100 and HY-100 steel weldments has been studied as a function of stress state using notched and unnotched axisymmetric tensile specimens. For the case of the HSLA-100 steel, the results for fine-grained, as well as coarse-grain HAZ (CGHAZ) material, show that, despite large differences in the deformation behavior when compared to base plate or weld metal, the failure strains are only weakly dependent on the thermal history or microstructure. Ductile microvoid fracture dominates the failure of the HSLA-100 steel with small losses of ductility occurring in the HAZ conditions only at high stress triaxialities. In contrast, the HY-100 steel is susceptible to a large loss of ductility over all of the stress states when subjected to a severe, single-pass simulation of a CGHAZ. The ductility loss is greatest at the high stress triaxiality ratio in which case failure initiation occurs by a combination of localized cleavage and ductile microvoid fracture.     

Effect of overaging on mechanical properties and stress corrosion cracking of HY-180M steel

The tensile properties, fracture toughness and stress corrosion cracking (SCC) behavior of HY-180 M steel at 22 °C were studied after final 5 h overaging treatments >510 ≤650 °C. SCC tests were conducted for 1000 h with compact tension specimens in aqueous 3.5 pct NaCl solutions at a noble (anodic) potential of −0.28 V_SHE ( −0.48 V_Ag/AgC1) and a cathodic protection potential of −0.80 V_SHE (−1.0 V_Ag/AgC1). The SCC resistance improved at aging temperatures >565 °C, the most significant improvement being at −0.80 V_She, especially after 650 ° aging whereK _ISCC was raised to at least 110 MPa · m^1/2. However, this was at the expense of mechanical properties. Provided low crack propagation rates of ∼3 X 10⁻¹¹ m/s at −0.80V _SHEmay be tolerated, the best compromise between strength, toughness, and SCC resistance was obtained after 594 °C aging. Under these conditions, stress intensities as high as ∼ 110 MPa · m^1/2 can be used, with a yield strength of ∼ 1150 MPa and fracture toughness of ∼ 170 MPa · m^1/2. The retained austenite content after aging increased with aging temperature up to 25 pct by vol at 650 °C. It appeared to correlate with improved SCC resistance, but other microstructural effects associated with aging may be involved. Formerly Research Associate with theDepartment of Metallurgical Engineering , University of BritishColumbia     

The microstructure and fracture of a carburized steel

The case microstructure and fracture of a coarse-grained 8620 steel carburized to 1 pet surface carbon are quite sensitive to austenitizing conditions. Reheating martensitic speci-mens below theA _cm produces in the case a refined austenitic grain size, a very fine mar-tensite, spherical carbide particles and a minimum of retained austenite and microcrack-ing. Overload fracture through the latter microstructure is transgranular and scanning electron microscopy shows both microvoid coalescence around thecarbide particles and an apparent fine cleavage in other areas. As-carburized specimens and specimens re-austenitized above theA _cm developed a case microstructure characterized by a coarse austenitic grain structure in which plate martensite with microcracks developed on cool-ing within a large amount of retained austenite. The overload fracture through this mi-crostructure followed a predominately intergranular path and effectively by-passed the retained austenite and microcracked martensite. Auger electron analysis showed that C and P were present on the intergranular fracture surfaces at concentrations above bulk, an observation consistent with literature reports of P segregation during austenitizing. This paper is based on a presentation made at a symposium on “Carburizing and Nitriding: Fundamentals, Processes and Properties” held at the Cincinnati Meeting of The Metallurgical Society of AIME, November 11 and 12, 1975 under the sponsorship of the Heat Treatment Committee.     

Effect of temperature upon stress corrosion cracking of HY-180M steel in 3.5 Pct NaCI

A fracture mechanics and fractographic study of stress corrosion cracking (SCC) of heat treated HY-180 M steel was undertaken over the temperature range 22 to 95 °C at applied potentials of −0.28 V_SHE (−0.48 V_Ag/AgCl) and −0.80 V_SHE (−1.0 V_Ag/Agcl). Particular attention was directed toward Region II behavior, where crack propagation rates were independent of stress intensity(K _l). Region II rates were always higher at the less noble potential of −0.80 V_SHE than at the more noble potential of — 0.28 V_SHE. However, fractography studies suggested that the basic mechanism of cracking at both potentials was the same, and involved hydrogen embrittlement. An Arrhenius analysis of Region II rates showed that crack propagation was under the control of more than one process. Consequently, the mechanistic details remained obscure. Formerly Research Associate in theDepartment of Metallurgical Engineering, University of BritishColumbia.     

Dynamic fracture initiation behavior of an HY-100 steel

An investigation was conducted into the effects of test temperature and loading rate on the initiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using fatigue precracked round bars loaded in tension to produce a quasi-static stress intensity rate of ·K₁ = 1 MPa√m/s and a dynamic rate of ·K₁ = 2 × 10⁶ MPa√m/s. Testing temperatures covered the range from -150 °C to 200 °C, which encompasses fracture initiation modes involving quasi-cleavage to fully ductile fracture. The results of toughness tests show that the lower-shelf values of fracture toughness were substantially independent of loading rate, while the dynamic values exceeded the quasi-static values by about 50 pct on the upper shelf. In analyzing these results, phenomenological fracture initiation models were adopted based on the requirement that, for fracture to occur, a critical strain or stress must be achieved over a critical distance. In separate tests, the observation of microfracture processes was investigated using fractography and anin situ scanning electron microscope (SEM) fracture technique. The layered ppearance of the fracture surfaces was found to be associated with a banded structure which generally contains many MnS inclusions, probably resulting in a reduction of the fracture toughness values.     

A fracture mechanics study of stress corrosion cracking of type-316 austenitic steel

A new test specimen configuration, designated the T-notch double cantilever beam (TNDCB), was developed, calibrated and employed for a fracture mechanics study of stress corrosion cracking (SCC) of cold worked Type-316 austenitic stainless steel exposed to hot aqueous solutions of 44.7 wt pct MgCl₂. The effects of stress intensity (K _I ), temperature (T) and electrochemical potential (E) upon the crack velocity (v) and fractography were investigated. The stress intensity (K _ISCC ) below whichv became immeasurably small was ∼12 MN·m^−3/2. Above this value, three regions of behavior were observed. Region I exhibitedK _I dependent cracking followed by Region II which exhibitedK _I independent cracking and an apparent activation energy of 63 to 67 kJ/mol, followed by Region III where cracking again became dependent uponK _I . The relative proportions of intergranular and transgranular crack paths were markedly dependent upon bothK _I andE, and less sensitive toT. Crack velocity was insensitive to small changes inE with respect to the free corrosion potentials (E _corr), but could be terminated by an applied active potential of ∼−0.35 V_SCE. The pH within the propagating crack was estimated to be <1.0 atE _corr, rising to ∼4.5 at −0.35 V_SCE. The mechanism of SCC was discussed with respect to film rupture events caused by crack tip plastic deformation, adsorption controlled processes on the metal surface, and hydrogen diffusion in the metal lattice. Alan J. RUSSELL, formerly Research Student, University of British Columbia     

微观组织对高速车轮钢解理断裂应力的影响

利用不同奥氏体化温度和冷却速率对碳质量分数为0.54%高速车轮钢进行热处理,得到具有不同晶粒尺寸和珠光体片间距微观组织的试样.在-120~20℃温度下对具有不同微观组织的拉伸试样和三点弯曲（3PB）缺口试样进行测试;采用二维平面应变有限元计算三点弯曲缺口试样缺口前的应力分布;利用扫描电镜对3PB试样断口进行观察并测量解理起裂源的位置;测定不同微观组织车轮钢试样的解理断裂应力.在扩展控制断裂机制下,微观组织对车轮钢的解理断裂应力具有明显影响,晶粒尺寸和珠光体片间距越小解理断裂应力越高.细化晶粒使未扩展微裂纹的特征长度减小,细化珠光体片间距有助于提高珠光体的有效表面能,从而使得解理断裂应力提高.     

The relationship between microstructure and fracture behavior of fully austenitic type 316L weldments at 4.2 K

Multipass weld deposits produced with a Mn-modified Type 316L filler material exhibited fracture toughness nearly 100 MPa√m less than that of a conventional 316L filler material when tested at 4.2 K. Although fracture in both materials occurred by ductile rupture, the crack path in the Mn-modified weld metal was microstructure-specific. The resultant fracture surface exhibited a “corduroy” morphology which reflected the underlying solidification pattern. Corresponding fracture surfaces in compact tension and tensile specimens from the standard 316L weld deposits showed little tendency for microstructure-specific fracture. A model is proposed which relates the fracture morphology and fracture toughness to the microstructural stability of the austenite during testing at 4.2 K. Partitioning of manganese and molybdenum to cellular dendritic boundaries during weld solidification tends to stabilize the austenite and suppress martensite formation in these regions. As a result, fracture occurs preferentially along these boundaries in the Mn-modified weld deposits, giving rise to the “corduroy” fracture morphology and providing less resistance to fracture than in weld deposits where martensite formation is more homogeneous. Formerly with Sandia National Laboratories, Livermore, CA 94550. Formerly of Lawrence Livermore National Laboratory.     

The influence of tensile stress states on the failure of HY-100 steel

The failure of an HY-100 steel plate has been examined as a function of stress state using notched and un-notched axisymmetric tensile specimens. The results show that increasing stress triaxiality leads to a rapid decrease in failure strains in a manner that is exponentially dependent on the degree of triaxiality. Two ductile failure mechanisms are identified: a void coalescence process, in which relatively equiaxed voids grow to impingement, and a void-sheet process, which links by a shear instability process large, elongated inclusion-initiated voids. The void-sheet mechanism intervenes and limits ductility at high-stress triaxialities in transversely oriented HY steel plate material, whereas the former process controls failure in longitudinally oriented material. These orientation effects are related to the morphology and alignment of the nonmetallic inclusion stringers that act as the primary void nucleation sites. Calcium treatments for inclusion-shape control improve ductility, especially at intermediate-stress triaxialities, primarily by suppressing the local conditions which give rise to the void-sheet instability process.     

The effects of microstructure,strength level,and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.     

Relation between microstructure, composition, and hot cracking in Ti-stabilized austenitic stainless steel weldments

A stabilized, fully austenitic alloy D9, a 15Cr-15Ni-2Mo stainless steel with a titanium addition corresponding to UNS 38660, is a candidate material for the fuel-clad and wrapper applications of the Prototype Fast Breeder Reactor (PFBR). The fully austenitic microstructure and the presence of titanium in this alloy lead to high susceptibility to hot cracking during welding. The fusion-zone and the heat-affected zone (HAZ) cracking susceptibility of alloy D9 was studied at three titanium levels, 0.22, 0.32, and 0.42 pct, all other elements remaining constant. The longitudinal and transverse Varestraint (Transvarestraint) hot-cracking tests were used to evaluate fusion-zone and HAZ cracking. The results showed that titanium increases cracking in the fusion zone by 15 to 20 pct in the range of Ti levels studied. The microanalysis of fusion-zone hot cracks using electron probe microanalysis (EPMA) showed an enrichment of Ti, C, N, and S along cracks and in the interdendritic regions. The corresponding phases were identified as TiC, TiC_0.3N_0.7, and the carbosulfides Ti₂CS and Ti₄C₂S₂, which are believed to form eutectics with austenite to produce cracking. The amounts of these phases increased with increasing Ti content. In the HAZ, a similar relation between titanium level and cracking was found. The comparison of the weldability of the D9 with an FA mode type 321 revealed that Ti-bearing eutectics were responsible for a high degree of cracking irrespective of the solidification mode. The results show that in the D9, the ratio of Ti to C and N must be controlled to minimize cracking.     

Influence of sulfide inclusion on ductility and fracture behavior of resulfurized HY-80 steel

The influence of sulfide inclusions on the ductile fracture process of experimental HY-80 steels having graded sulfur levels from 50 to 500 ppm and heat-treated to different strength levels was studied with respect to mechanical properties, namely, tensile ductility and Charpy impact en-ergy. Sulfide inclusions are found to have deleterious effect on both axisymmetric ductility and Charpy impact properties, whereas the plane strain ductility was found to be less sensitive to sulfide inclusions. The effect of interaction between the inclusion and the matrix and the as-sociated stress strain distribution at the void nucleating sites, which control the fracture process by microvoid coalescence, were discussed in the light of various models to suggest a micro-mechanism of fracture. Other toughness parameters obtained from instrumented impact tests were evaluated and discussed as a function of sulfur content.     

Stress corrosion cracking of high strength HY-180M steel in 3.5 Pct NaCl

Stress corrosion cracking of HY-180M steel was studied at 22°C in an aqueous solution of 3.5 pct NaCl (pH = 6.5). The steel had a nominal weight percentage composition of 10Ni-14Co-2Cr-lMo-0.16C and was heat treated to yield a fracture toughness value ofK _Ic ≃ 160 MPa . m^1/2. The SCC velocity (v) was studied as a function of stress intensity (K _I) and electrochemical potential (E) using precracked compact tension specimens, a Ag/AgCl reference electrode and a 1000 h exposure test. Also, the polarization behavior, microstructure, fractography and corrosion products were studied. The results showed that SCC was markedly dependent uponE, and did not occur whenE =-0.52 V_SHE (-0.72 V_Ag/AgCl), which corresponded closely to the thermodynamically reversible potential of iron. However, SCC occurred at a more noble potential of-0.28 V_SHE (-0.48 V_Ag/AgCl ) and at a less noble potential of-0.80 V_SHE (-1.00 V_Ag/AgCl). The stress intensity below which SCC was not observed was K_ISCC ≃ 5.5 MPa . m^1/2 at -0.28 V_SHE and K_ISCC ≃ 60 MPa . m^1/2 at -0.80 V_SHE . Also, Region I behavior (v dependent uponK ₁) and Region II behavior (v independent ofK ₁) were observed. Cracking was considered to occur solely by hydrogen embrittlement at -0.80 Vshe, whereas anodic dissolution processes played a necessary role, either directly or indirectly, in SCC at -0.28 V_SHE . The indirect effects were discussed in relation to hydrolysis effects in the crack promoting hydrogen embrittlement and/or corrosion product wedging stresses.     

Hydrogen-facilitated corrosion and stress corrosion cracking of austenitic stainless steel of type 310

The effects of hydrogen précharge and stress on anodic dissolution for Type 310 austenitic stainless steel (ASS) have been investigated. An experiment determining the effect of hydrogen on stress corrosion cracking (SCC) was carried out in a boiling 42 pct MgCl₂ solution and in a 2.5 mo/L H₂SO₄ + 1 mol/L HC1 solution. The results showed that both hydrogen and stress would increase the dissolution rate, and the effects of hydrogen and stress on the dissolution rate were synergistic rather than simply additive. Hydrogen lowered the threshold stress and the shortened fracture time of SCC in a boiling MgCl₂ solution by a factor of 1/5 and 10, respectively.     

The effect of potential and aging on the Pb-assisted stress corrosion cracking susceptibility of alloy 22 gas tungsten arc-welded weldments

The susceptibility of as-received, solutionized, and short-term thermally aged mill-annealed (MA) and gas tungsten arc-welded (GTAW) alloy 22 to Pb-assisted stress corrosion cracking (PbSCC) was evaluated in supersaturated, deaerated, acidic PbCl₂ solutions at 95 °C. Anodic polarization tests in acidic PbCl₂ solutions showed that 16,000 ppm of Pb produced a strong anodic peak and an order of magnitude greater passive current density for both MA and GTAW alloy 22 as compared to pure NaCl solutions. Current spikes were also observed in the anodic polarization plots for the PbCl₂ solutions, suggesting periodic events of passivity breakdown and repassivation. Constant deformation SCC tests were conducted using double U-bend samples of as-received, solutionized, and thermally aged MA and double U-groove welded alloy 22 plates. The results indicate that as-received, solutionized, and thermally aged MA and GTAW alloy 22 were resistant to PbSCC in supersaturated PbCl₂ solutions at 95 °C, pH 0.5, and applied potentials near the anodic peak ranging from −100 to 50 mV_SCE. Enhanced dissolution of alloy 22 was also observed in the crevice region of the double U-bend samples tested in the 16,000 ppm PbCl₂ solutions. This Pb concentration is seven orders of magnitude greater than that found in the anticipated repository environments, and chemical speciation modeling showed that Pb²⁺ is strongly immobilized in J-13 Yucca Mountain waters through the precipitation of PbCO₃ solids. Therefore, although enhanced dissolution of the inner U-bend did occur in our tests, the overall results from this PbSCC investigation suggest that as-fabricated, solutionized, and aged MA and GTAW alloy 22 are resistant to SCC in extremely aggressive, acidic, and supersaturated PbCl₂ solutions at 95 °C. Provided that these high Pb concentrations are not attainable in the anticipated repository environments, alloy 22 is unlikely to be susceptible to SCC, localized corrosion, and enhanced dissolution by the presence of Pb. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.