The Tensile Properties of an Fe-4 Pct Mo-0.2 Pct C Martensite Tempered under Applied Stress

In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure occurred in tensile test at 600 °C to 700 °C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered under applied stress and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular fracture was observed in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes to 10 minutes and applied stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused by segregation of P, S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress was found to be very significant. Namely, when the tensile direction was parallel to the applied stress during tempering, the specimen was more brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle. (4) To reduce this embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was considerably reduced both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain boundaries. Formerly with Kyoto University Formerly with Kyoto University This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Stress corrosion cracking of alpha brass in a tarnishing ammoniacal environment: Fractography and chemical analysis

Alloy 260 brass specimens under stress were exposed at room temperature to 15 N aqueous ammonia solution with 8 g/1 of cupric copper predissolved. This environment causes tarnishing of the brass surface and intergranular stress corrosion cracking. Scanning electron microscopy, energy dispersive X-ray analysis, and Auger electron spectroscopy were employed to study fractography, corrosion product composition and distribution within the stress corrosion crack, and fracture surface chemistry characteristic of stress corrosion cracking in this system. A thin oxidized film was detected by Auger spectroscopy at the leading edge of the propagating crack. With continued exposure to the corrosive environment, deposits form on the fracture surface, then coalesce to form a continuous tarnish film that is depleted of zinc. No bulk depletion of zinc was detected in the alloy at the stress corrosion crack leading edge. No evidence of noncrystallographic crack arrest marks was found on the intergranular fracture surface.     

Combined effects of pb and sn additions on the corrosion of Zn-12 Pct Al-1 Pct Cu-0.02 Pct Mg sand cast alloy in wet steam

The effects of varying the Sn content from 0 to 0.01 pct at 0.005 and 0.015 pct Pb on the corrosion of sand-cast Zn-12 pct Al-1 pct Cu-0.02 pct Mg alloy in saturated water vapour at 95°C were investigated. Within the relevant specification limits there was no interaction between Pb and Sn, the combined effect being equivalent to that of Sn alone. The tolerance of the alloy for Sn was so low that the maximum for this impurity was considered to be 0.001 pct. For Pb, 0.01 pct produced substantially the same corrosive attack as 0.001 pct and would be considered as the upper limit for this alloy.     

Fatigue threshold and low-rate crack propagation properties for structural steels in 3 Pct sodium chloride aqueous solution

The fatigue threshold and low-rate crack propagation properties for a carbon steel, two high-strength steels, and two stainless steels were investigated in a 3 pct sodium chloride aqueous solution at frequencies between 0.03 and 30 Hz. Tests were conducted in a manner designed to avoid crack closure. Under freely corroding conditions, the effective values of the threshold stress intensity factor range, ΔK_th,eff, were lower than in air for all of the steels. In particular, the ΔK_th,eff values for the carbon and high-strength steels were almost equal to the theoretical ΔKth value of about 1 MPa m^1/2 calculated on the basis of the dislocation emission from the crack tip. At a given ΔK level higher than the threshold, the fatigue crack propagation rates accelerated with decreasing frequency for all of the steels. Under cathodic protection, the threshold and fatigue crack propagation properties were coincident with those in air regardless of material and frequency. The observed fatigue crack propagation behavior in a 3 pct NaCl solution was closely related to the corrosion reaction of the bare surface formed at the crack tip during each loading cycle.     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

Effect of cold work on stress corrosion cracking behavior of types 304 and 316 stainless steels

The influence of cold work (prestraining) in the range 2.3 to 56 pct on stress corrosion cracking (SCC) properties of types 304 and 316 stainless steels in boiling MgCl2 solution at 154 °C was investigated using a constant load method. In both materials, SCC initiation was in transgranular mode. Transition in stress corrosion cracking mode from transgranular to intergranular, as the crack proceeds, was observed at all cold work levels in 316 stainless steel and at cold work levels of 26 pct and 56 pct in 304 stainless steel. Both prestraining and increase in the initial applied stress facilitated the transition in crack morphology to intergranular mode. Increased tendency to intergranular SCC at high applied stresses and in cold worked specimens appears to be mechanistically analogous.     

The stress corrosion resistance and fatigue crack growth rate of a high strength martensitic stainless steel,AFC 77

The plane-strain stress corrosion thresholdK _Iscc and fatigue crack growth rate have been determined for a high strength martensitic stainless steel, AFC 77, in both conventionally processed and strain-aged conditions. TheK _Iscc (in 3.5 pct sodium chloride solution) is markedly affected by both the tempering temperature and the degree of strain aging. The highestK _Iscc of 105 ksi \(\sqrt {in} \) . was obtained by tempering at 500°F and the lowestK _Iscc of 10 ksi \(\sqrt {in} \) . by tempering at 1100°F. Retained austenite raisedK _Iscc at tempering temperatures up to 1000°F, which was the highest tempering temperature at which austenite could be maintained. Fatigue crack growth rates in both dry air (<10pct relative humidity) and 3.5 pct sodium chloride solution were at a maximum for material tempered at 700°F. Over the range of stress intensity studied, retained austenite reduced fatigue crack growth rate in salt solution but increased it in dry air.     

Effect of cold work on stress corrosion cracking behavior of types 304 and 316 stainless steels

The influence of cold work (prestraining) in the range 2.3 to 56 pct on stress corrosion cracking (SCC) properties of types 304 and 316 stainless steels in boiling MgCl2 solution at 154 °C was investigated using a constant load method. In both materials, SCC initiation was in transgranular mode. Transition in stress corrosion cracking mode from transgranular to intergranular, as the crack proceeds, was observed at all cold work levels in 316 stainless steel and at cold work levels of 26 pct and 56 pct in 304 stainless steel. Both prestraining and increase in the initial applied stress facilitated the transition in crack morphology to intergranular mode. Increased tendency to intergranular SCC at high applied stresses and in cold worked specimens appears to be mechanistically analogous.     

Fatigue in Binary Alloys of bcc Iron

The effects of twelve alloying elements on the fatigue properties of α-iron were determined. Stress-life properties, fatigue crack propagation (FCP) rates, and stress intensity thresholds of 38 kinds of binary ferrite containing an alloying element varying from 1 to 5 wt pct or up to its solubility limit in α-iron were experimentally determined together with those of the base iron. It was shown that the influence of a given element on high cycle fatigue life differed from that of its effects on FCP resistance or on stress intensity thresholds.     

Influence of texture on fatigue properties of Ti-6Al-4V

Tensile properties, high cycle fatigue strength, and fatigue crack propagation behavior were evaluated on highly textured Ti-6Al-4V material to investigate the influence of a preferred crystallographic orientation on mechanical properties. Thermomechanical treatments were used to develop three different textures: a basal, basal/transverse, and transverse type, all of which exhibited the same homogeneously equiaxed microstructure. The Young’s modulus was found to vary between 107 and 126 GNm^-2, and yield strength changed from 1055 to 1170 MNm^-2. Ductility was only slightly affected by texture. High cycle fatigue and fatigue crack growth measurements were performed in vacuum, laboratory air, and a 3.5 pct NaCl solution. It is shown that laboratory air can be regarded as a quite corrosive environment. In vacuum the highest fatigue strength values were measured whenever loads were perpendicular to basal planes. However, these conditions had the highest susceptibilities to air and 3.5 pct NaCl solution environments. Nearly no influence of texture on fatigue crack propagation was found in vacuum, but in a corrosive environment crack growth parallel to (0002)-planes was much faster than perpendicular to these planes. To explain the corrosive effect on the fatigue properties of the textured material hydrogen is thought to play a key role.     

Stress Corrosion Cracking of HY-180 Steel in Aqueous 3.5 Pct NaCI

Stress corrosion cracking of HY-180 steel (Fe-10 Ni-2 Cr-1 Mo-8 Co-0.12 C) was studied in aqueous 3.5 pct NaCI (pH = 6.5) at 22 °C. The alloy was austenitized, water quenched and aged at 510 °C for 5 h. Specimens were of the precracked, double cantilever beam (DCB) variety and exposure times extended up to 1000 h. The crack propagation rates (v) were studied as a function of stress intensity(K,) under both freely corroding potentials(E ≈-0.36 V_SHE) and potentials produced by coupling to Zn(E ≈ -0.82 V_SHE. Crack fractography was studied by scanning electron microscopy and corrosion products were identified by electron diffraction analysis. The stress intensity, K_ISCC, below which SCC could not be detected was ~45 MPa m^1/2 for both freely corroding and Zn-coupled conditions. Analysis of the results showed that cracking was consistent with a hydrogen embrittlement mechanism, irrespective of potential. Furthermore, comparison of the data with previous studies on a similarly heat treated and closely related alloy (HY-180 M), containing 14 Co-0.16 C, showed no significant difference in SCC behavior, provided comparison was made at similar electrochemical potentials.     

Effects of sodium chloride and shot peening on corrosion fatigue of AISI 6150 steel

Reversed-bending fatigue tests of quenched and tempered AISI 6150 steel were conducted in dry air and in aqueous 3 pct NaCl. The 3 pct NaCl environment drastically reduced fatigue life but two different shot peening treatments were found to improve the corrosion fatigue life over that of unpeened samples. Multiple fatigue crack initiation occurred at very distinct locations in both the unpeened and the peened specimens fatigued in 3 pct NaCl. Fatigue crack propagation from each initiation site occurred first on flat facets normal to the stress axis and then by a more ductile mechanism after the initiation facets had linked. The average size of the corrosion fatigue initiation facets in the peened specimens was much smaller than that of the unpeened specimens; however, the number of initiation sites was greater in the peened specimens. It is believed that the beneficial effect of the shot peening results from significantly reduced early fatigue crack propagation rates in the compressive residual stress layer at the surface. Formerly Research Assistant, Materials Department, University of Wisconsin-Milwaukee     

Stress-corrosion cracking behavior of an 18 Pct Ni maraging steel

Stress-corrosion cracking of an 18 pct Ni maraging steel in aqueous solutions was studied using precracked cantilever beam specimens. By appropriate heat treatments, six different structures having the same yield strength were obtained. Although significantly different plane strain fracture toughness values (K _Ic ) resulted, it was found that the threshold plane strain stress intensity (K _Iscc ) was the same for all structures.K _Iscc had the same value in 3 pct NaCl at various pH values, in 1N H₂SO₄, and in distilled water. Specimens tested in 3 pct NaCl under both anodic and cathodic applied potentials also exhibited this sameK _Iscc value. Fractographic inspection of the crack surfaces revealed no apparent differences due to changes in solution, pH, or applied potential. The crack path was intergranular in all cases. However, specimens austenitized at 1500°F exhibited crack branching, whereas in specimens austenized at much higher temperatures branching no longer occurred. Aging time and temperature seemed to change only the time to failure. The mechanism most consistent with all observations appears to be hydrogen cracking.     

Fatigue of Cu-1 Pct Cd

The fatigue properties of high conductivity Cu-1 pct Cd are reported. In contrast to Cu, the fatigue strength of Cu-Cd is grain-size dependent in high cycle fatigue. Strain aging is responsible for the superior fatigue properties of Cu-Cd, and the effect of dislocation pinning is carried over to the cold work state, giving a work hardened structure which is stable in fatigue loading. A notable feature of the results is that fatigue fracture in Cu-Cd is almost entirely intergranular at long fatigue lives, in contrast to the fracture in tensile loading and in contrast to the transgranular fracture which was observed in Cu. The effect is due to segregation of Cd at the grain boundaries and to the effect of Cd on the cohesive strength of the boundary. Moderate cold work suppresses intergranular fracture. Formerly Department of Metallurgy and Engineering Materials, The University of Newcastle upon Tyne     

The effect of microstructure on the mode of monotonie fracture,fatigue crack initiation,and stage i crack growth in an Fe-21 Pct Cr-11 Pct Ni heat resisting steel

This paper describes a study carried out at room temperature on an Fe-21 pct Cr-11 pct Ni heat resisting alloy under tensile and fatigue deformation. Specific microstructures were developed by heat treating the as-received alloy at different temperatures and times. The surface condition of all specimens displayed surface grain boundary oxidation to a maximum depth of 0.16 mm. In addition, the microstructure of specimens in one batch (B) contained intergranular chromium carbides. The major conclusions drawn from this study are that different microstructures respond differently to monotonie and cyclic modes of deformation. In particular, the embrittling effect of intergranular chromium carbides observed during the monotonie mode of deformation was different from that found when deformation was cyclic. During cyclic deformation these chromium carbides assisted in reducing the damaging effects of the surface grain boundary oxidation. Also during cyclic deformation, the overall fatigue life was found to depend on the mode of both fatigue crack initiation and Stage I crack growth. Fatigue life was reduced when crack initiation and Stage I crack growth were intergranular while it was enhanced when crack initiation occurred at slip bands and subsequent Stage I crack growth was transgranular. It was observed that surface grain boundary oxidation is a most deleterious micro-structural feature especially under fatigue loading but, if this feature is unavoidable then the presence of intergranular chromium carbides is considered to be highly beneficial in increasing the overall fatigue resistance of the material. Formerly a Postgraduate Student, School of Materials Science and Engineering, University of New South Wales, Kensington, New South Wales 2033.     

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.     

On the corrosion fatigue behavior of a modified SAE 4135 type steel in a H2S environment

The corrosion fatigue behavior of a modified SAE 4135 steel in a H₂S environment is compared to that of a standard SAE 4135 steel of the same strength level. The modification consists of an increase in molybdenum content from 0.20 to 0.75 pct and the addition of 0.036 pct columbium. It was found that the high cycle fatigue strength of the modified steel was 66 pct higher than that of the standard steel. The improvement is attributed to an increase in resistance to hydrogen embrittlement brought about by a higher tempering temperature which reduces the amount of phosphorous on grain boundaries, as well as the beneficial scavenging effects associated with the increase in molybdenum content and the presence of columbium. Formerly Graduate Student at University of Connecticut, is now with Nippon Kokan, K. K., Fukuyama, Japan.     

A study of the mechanism of stress-corrosion crack propagation in AISI 4340 steel in aqueous 3.5 wt.% NaCl solution

Stress-corrosion (SC) crack propagation in AISI 4340 steel has been studied with 2 mm thick single edge-notched (SEN) specimens under constant load conditions as a function of applied potential and tempering conditions in an aqueous 3.5 wt.% NaCI solution at 30°C. The SC crack lengths were estimated by using the electrical potential method. As the amount of cathodic polarization increased, the SC crack propagation rate increased. Anodic polarization yielded opposite results. These polarization effects on the SC crack propagation are discussed in terms of absorbed hydrogen resulting from a cathodic reaction on the specimen surface. SC cracks propagated by intergranular fracture through most of the inner region, but shear lips were formed at the near subsurface, irrespective of applied potential and tempering temperature. This is explained in terms of the stress state dependency of hydrogen behaviour. Above experimental evidence well supports the theory that SC crack propagation is controlled by the hydrogen embrittlement (HE) process. The SC crack propagation rate decreased in the sequence of 300, 200, and 400°C-tempered specimens. This is discussed as being related to the microstructural and yield stress effects.     

The influence of oxygen on the structure,fracture, and fatigue crack propagation behavior of Ti-8.6 Wt Pct Al

The effect of interstitial oxygen content on the structure, fracture, and fatigue crack propagation (FCP) behavior of Ti-8.6 wt pct Al was studied. Increasing oxygen was seen to raise the tensile flow strength of Ti-8.6A1 while coincidentally decreasing fracture ductility due to a transition in fracture mode from ductile rupture to cleavage or slipband fracture. The influence of oxygen onda/dn vs ΔK of Ti-8.6A1 in vacuum was found to vary with the R ratio. At R = 0.1, increasing oxygen led to slightly reduced FCP rates, while at R = 0.7, increasing oxygen from 500 to 1000 or 2000 ppm increased FCP rates. TheK _Ic of precipitation-aged Ti-8.6A1 was seen to be reduced with increasing oxygen. The detrimental influence of oxygen on fracture ductility andK _Ic is linked to oxygen’s influence on increasing slip planarity, particularly in the presence of α₂. The influence of deformation character (in particular, strain localization), rather than flow stress, in promoting cleavage-like fracture is discussed.