Reduction in area and appearance of hydrogen-assisted fracture surfaces of cathodically charged differnt Low-alloy sulphur containing steels

The present work is concerned with the role of sulphidic inclusions in hydrogen-assisted cracking of low-alloy steels with different sulphur contents. The steels were previously cathodically charged in sulphuric acid solution. Evaluating the susceptibility to hydrogen-assisted cracking in terms of the reduction in area values, the high sulphur steel is less susceptible than the low sulphur steel. The fracture surface of the low sulphur steel appears rather brittle, with local quasi-cleavage fractures around large intergranular fracture facets. In contrast, the fracture surface of the high sulphur steel is characterized predominantly by the microvoid coalescence mode, with a great number of manganese sulphide inclusions. The difference between the two sulphur steels is based upon the concept that the interfaces between sulphidic inclusions and matrix act as trapping sites for hydrogen and thus hydrogen is uniformly distributed over the sulphidic inclusions.     

Untersuchungen über den Einfluß des Zinks auf das Korrosionsverhalten von Spannstählen

Investigations on the influence of zinc on the corrosion behavior of high strength steels Corrosion and fracture behavior of hot dip galvanized high strength steels with and without damaging zinc layer was studied in deionized water, in tap water, and in saturated calcium hydroxide solution through which nitrogen, air, and carbon dioxide were bubbled separately. Hydrogen permeation measurements have been carried out for steel specimens which were cathodically polarized by means of the zinc layer. It has been observed that the hydrogen activity reached to a maximum of 1-2 in neutral solutions and 4-10 in saturated calcium hydroxide solution or in dilute acid solutions containing carbon dioxide, before decreasing to low values. The decrease of hydrogen activity is due to the formation of surface layers. Constant load tests by application of a load amounting to 100-110% yield strength caused no hydrogen induced brittle fracture within 300-400 h neither with nor without damaging zinc layer. At a constant strain rate of 5· 10^?7/s only 8 mm cold drawn steel wires were found to be almost unsusceptible to hydrogen that was absorbed during the test. Under notched conditions, all the steels investigated showed macroscopic brittle fracture.     

A hydrogen embrittlement mechanism for sensitized types 304, 316 and 310 austenitic stainless steels in boiling saturated magnesium chloride solutions

We have already proposed a mechanism for intergranular hydrogen embrittlement (IG-HE) for solution annealed austenitic stainless steels (types 304, 316 and 310) in HCl solutions and in boiling saturated magnesium chloride solutions. The proposed IG-HE mechanism was based on martensite transformation, hydrogen-enhanced local plasticity (HELP), grain boundary sliding (GBS). Recently, it was reported that the fracture susceptibility and fracture mode for sensitized steels in boiling saturated magnesium chloride solution under an open-circuit condition were significantly different from those observed for solution annealed steels. In the present paper, the hydrogen embrittlement behavior of sensitized types 304, 316 and 310 in boiling saturated magnesium chloride solutions was explained in more details in terms of an inhibiting effect of chloride ions, martensite transformation, Cr depletion, HELP, the degree of corrosiveness through the comparison with those for the solution annealed steels. Furthermore, a transgranular HE (TG-HE) cracking mode that was not observed for the solution annealed steels was discussed as well as IG-HE. Then a TG-HE mechanism for sensitized austenitic stainless steels was proposed, while the IG-HE mechanism for solution annealed austenitic stainless steels which was discussed in details was applied to IG-HE of sensitized austenitic stainless steels. It was also pointed out that the occurrence of both TG-HE and IG-HE was explained with an identical concept.     

新型1500MPa级高强钢的氢脆敏感性研究

根据钢材组织设计的思想，通过优化成分和工艺设计、研制出一种新的1500MPa级高强纲。采用阴极电解充氢的方法对其氢脆敏感性进行了研究，并与同一强度级别的42CrMo高强钢进行了对比。结果表明，所设计的1500MPa级高强钢的氢脆敏感性低于传统的42CrMo高强钢。SEM断口观察显示，两者的断口形貌也不同，1500MPa级高强纲为准解理断裂，而42CrMo高强钢为沿晶断裂。断口金相表明，前者的裂纹主要沿着贝氏体/马氏体（B/M）边界扩展，断裂模式为板条界分离，后者的裂纹沿着晶界扩展。对1500MPa级高强钢进行了TEM观察，发现其组织为贝氏体/马氏体复相组织，残留奥氏体以薄膜状存在贝氏体内部及贝氏体条片、马氏体板条间。     

1500 MPa级40CrNi3MoV钢的氢脆敏感性

通过慢应变速率拉伸实验研究了Si含量分别为0.25%和1.16%的1500 MPa级40CrNi3MoV钢的氢脆敏感性,即充氢后缺口试样抗拉强度下降率,冲击实验用来测试1 mA/cm2电流密度下充氢后试样的断裂韧性值,分析氢致裂纹的扩展方式.结果表明,由于Si抑制回火过程中碳化物的形核和长大,高Si含量的40CrNi3MoV钢中回火析出的碳化物被细化且弥散分布,作为氢陷阱使氢分布均匀,抑制了氢向裂纹尖端扩散,高Si含量的40CrNi3MoV钢的氢脆敏感性较低.     

Evaluation of susceptibility of high strength steels to delayed fracture by using cyclic corrosion test and slow strain rate test

To evaluate susceptibilities of high strength steels to delayed fracture, slow strain rate tests (SSRT) of notched bar specimens of AISI 4135 with tensile strengths of 1300 and 1500 MPa and boron-bearing steel with 1300 MPa have been performed after cyclic corrosion test (CCT). During SSRT the humidity around the specimen was kept high to keep absorbed diffusible hydrogen. The fracture stresses of AISI 4135 steels decreased with increment of diffusible hydrogen content which increased with CCT cycles. Their delayed fracture susceptibilities could be successfully evaluated in consideration of both influence of hydrogen content on mechanical property and hydrogen entry.     

Potentialabhängigkeit der H-induzierten Korrosion (HIC) und H-induzierten Spannungsrißkorrosion (HSCC) bei Röhrenstählen in schwach sauren und neutralen Medien

Influence of Potential on Hydrogen-Induced Cracking (HIC) and Hydrogen-Induced Stress Corrosion Cracking (HSCC) of Line-Pipe Steels in Weak Acidic and Neutral Environments In weak acidic (pH 5.5), sulfide-containing environments, high-strength line-pipe steels (0.2% yield strength 430 to 700 Nmm^?2) show a critical mechanical stress level σ_cr ≈ 60% YS above which HSCC occurs. The given value is independent of the mechanical properties. σ_cr decreases with decreasing pH (pH 3: σ_cr ≈? 30% YS), increases with increasing pH and becomes identical with the YS at sufficiently high pH. In buffer solution pH 5.5, σ_cr corresponds to a critical limiting potential for the occurence of HSCC, which depends on the mechanical stress level, ranging between U_H = ?0.42 and ?0.55 V for σ = 60% YS. At potentials more negative than a cathodic limiting potential, which ranges from U_H = ?0.8 to ?1.0 V, no HSCC occurs. This is probably due to the formation of protective layers or of an alkaline environment on the surface. This limiting potential is shifted into the positive direction with decreasing pH and does no more exist in acidic environments. With anodic polarization to potentials more positive than the free corrosion potential, uniform attack occurs resulting in failures of the specimens by reduction of area. Susceptibility to HSCC has a maximum at ambient temperature. There exists also a critical potential U_HIC for the occurrence of longitudinal cracks due to HIC which is more positive than U_HSCC. There is no connection found between U_HIC and U_HSCC nor between σ_cr and U_HIC. From the occurrence of HIC one can not derive any conclusions for the susceptibility or resistance of the given material to HSCC. As a summary, high strength line-pipe steels display no higher susceptibility to HSCC in sulfide-containing media than steels with minor mechanical properties. The HSCC-behaviour of the steels under investigation in buffer solution pH 5.5 is completely equivalent to the behaviour in stronger acidic environments (pH 3).     

Effect of phosphorus segregation on fracture properties of 2.25Cr-1Mo pressure vessel steel

Phosphorus is a very common trace element that can segregate at prior austenite grain boundaries and/or carbide/matrix interfaces of low alloy steels at high temperature (e.g., order of 500 °C) and adversely affect the fracture properties. This paper investigates segregation of P during reversible temper embrittlement (96 h at 520 °C) of quenched and fully tempered 2.25Cr-1Mo steel by Auger electron spectroscopy and describes the segregation mechanism. This paper also describes the effect of P segregation on fracture resistance and fracture mode of unembrittled steels, respectively, by fracture toughness testing over a temperature range of −196 °C to 20 °C and fractography in scanning electron microscopes. During temper embrittlement phosphorus segregation has been attributed due to the mechanism of “carbide rejection”. This segregation caused a reduction in fracture toughness values of the quenched and tempered steels at all test temperatures and an increase in the transition temperature. Phosphorus segregation also changed the brittle fracture micromechanism of quenched and fully tempered samples from one of transgranular cleavage to a mixed mode of fracture (transgranular cleavage and intergranular decohesion). The micromechanism of fracture at temperatures from the upper shelf, however, remained almost unchanged.     

Vergleich von Bauteil- und Laboratoriumsversuchen im System Baustahl-H2S-haltige,feuchte Gase bzw. wäßrige Prüflösungen

Full scale tests and laboratory tests of low alloy steels in wet and liquid H₂S containing environments The results of five hydrogen induced cracking(HIC)-Round Robin tests of steels exhibiting different levels of HIC susceptibility involving nine laboratories and a smaller number of hydrogen induced stress corrosion cracking (HSCC)-Round Robin tests are presented. These results are compared with the results of full scale tests using the same steels and the same liquid environment as in the Round Robin tests. This full scale tests were followed by full scale tests with humidified gaseous environments saturated with H₂S and CO₂.     

Effect of structure and strength on the corrosion resistance of corrosion-resistant steels and alloys in media containing hydrogen sulfide and chlorine ions

Equipment in gas and oil wells comes in contact with media, whose aggressiveness is determined by the chlorine-ion content, the partial pressure of hydrogen sulfide and carbon dioxide gas, and temperature. Hydrogen sulfide corrosion cracking (HSCC), pitting, and general corrosion are basic forms of corrosion failure in these inedia. In a number of cases, special structural steels, whose corrosion resistance can be enhanced with inhibition of the medium, are used in fabricating the equipment. Corrosion-resistant steels should be employed when inhibition is inapplicable, and, in addition to HSCC, the development of pitting and general corrosion is possible during service. The resistance of austenitic, austenitic -ferritic, and rnartensitic steels to HSCC and pitting are investigated in this paper.I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 20–25, July, 1994.     

阴极保护对低合金钢在海水中断裂行为的影响

本文系统地研究了AISI4340,HY100,WELTEN60,AISIA537和A131低合金钢在开路和阴极保护电位下海水中的应力腐蚀开裂(SCC)和腐蚀疲劳(CF)行为.试验结果表明,阴极保护可以使合金钢在海水中不发生SCC的界限强度降低;而对低合金钢在海水中CF裂纹扩展速率的影响则不是单一的,与其强度及SCC的行为密切相关.指出在海洋工程结构中,应尽可能把阴极保护电位控制在规定区域的上限,同时可把海洋平台用钢的强度提高200—300MPa     

Evaluation of delayed fracture property of outdoor-exposed high strength AISI 4135 steels

Delayed fracture properties of AISI 4135 high strength steels with 1490 and 1310 MPa of tensile strength, represented as B15 and B13, respectively, have been studied by means of slow strain rate test (SSRT) of notched bar specimens after outdoor exposure at rural and coastal areas. The exposed specimens were kept at humid medium before SSRT to reproduce active hydrogen entry influenced by the rust layer and to homogenize hydrogen distribution. The influences of exposure site and exposure time on fracture stress have been investigated. The susceptibility of B15 to delayed fracture was obviously higher than that of B13.     

Hydrogen embrittlement susceptibility and permeability of two ultra-high strength steels

Slow displacement rate tensile tests were carried out in a saturated H₂S solution to investigate the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of two ultra-high strength steels (PH 13-8 Mo stainless steel and T-200 maraging steel). Hydrogen permeation properties were determined by an electrochemical permeation method. The results of permeation tests indicated that over-aged specimens showed a lower diffusivity/hydrogen flux and higher solubility than those solution-annealed. The great increase in reverted austenite (irreversible hydrogen traps) together with numerous precipitates at the expense of dislocations (reversible) in the over-aged specimen led to such a change in permeability. Ordinary tensile tests indicated that four tested specimens had roughly the same yield strength level. Hence, the hydrogen embrittlement susceptibility of the material could be related to their permeation properties. The uniform distribution of strong hydrogen traps in over-aged specimens instead of weak traps in the solution-annealed impeded the hydrogen transport toward the strained region, thus, the resistance to sulfide stress corrosion cracking was improved in over-aged specimens.     

EFFECT OF CATHODIC PROTECTION ON FRACTURE BEHAVIOUR OF LOWALLOY STEELS IN SEAWATER

Fracture behaviour of low-alloy steels AIST4340,HY100,Welten60,AISIA537 and A131 inartificial seawater under static,cyclic loading and at cathodic protection potential has beeninvestigated by using the techniques of fracture mechanics, electrochemistry andelectronfractography.The results reveal that at hydrogen evolution potentials(cathodic pro-tection potential)the critical yield strength required for the occurrance of SCC decreasesfrom 985 at corrosion potential(E_c)to 872 MPa.The effect of cathodic protection on crackpropagation of corrosion fatigue(CF)is not simple,it is closely related to the yield strengthof steels and their SCC behaviour.For the steels with high yield strength,cathodic protectionpromotes(da/dN)_Ⅱ evidently,and reduces △K_(th) value.The(da/dN)-△K curves dis-play a plateau at the third stage of CF for steels with medium or low yield strength.It is men-tioned that the cathodic protection potential for oceaneering constructures should be control-led at top level of the protective range.It seems reasonable that the strength of steel foroceaneeing use might be increased by 200 or 300 MPa.     

Cold cracking susceptibility of boron added high-strength bainitic steels

This study evaluated the cold cracking susceptibility of high-strength bainitic steels having a strength level of over 800 MPa and 1.1 GPa according to their respective chemical compositions. For evaluation of cold cracking susceptibility, a modified implant test was employed. Two different shielding gases (100% CO₂ gas and 98% CO₂ mixed with 2% H₂ gas) were used to estimate the effect of diffusible hydrogen on the cold cracking. The diffusible hydrogen contents measuring 100% CO₂ and 98% CO₂+2% H₂ gas were recorded as 0.58 and 9.01 ml/100 g, respectively. In the case of the lower hydrogen content, cold cracking susceptibility was very low in spite of the very high strength of the base steel regardless of chemical composition and rolling condition. However, when the mixed shielding gas was used, cold cracking susceptibility increased due to an increase in the hydrogen content. Most notably, the cold cracking resistance of steels containing higher alloy content deteriorated significantly due to the higher hardness of the coarsegrained heat-affected zone. However, the effect of the rolling condition on the cold cracking susceptibility turned out to be negligible compared to that of the chemical composition.     

Corrosion behaviour of experimental copper–antimony–molybdenum carbon steels in industrial and marine atmospheres and in a sulphuric acid aqueous solution

Low alloyed carbon steels are used in several applications as in automotive, home appliances and civil industries. Sb-bearing steels have been developed to withstand acid condensation, mainly to exhibit corrosion resistance to sulphuric acid aqueous solutions. This work is aimed at studying the corrosion resistance of three experimental low alloyed carbon steels with additions of copper, antimony and molybdenum using the electrochemical impedance spectroscopy (EIS) in a sulphuric acid aqueous solution, and field tests in industrial and marine atmospheres. The field tests showed the mass loss of antimony–molybdenum carbon steels was higher compared to that of other steels. The alloyed carbon steels with copper and antimony additions showed the highest atmospheric corrosion resistance evaluated by using field tests in industrial and marine environments. The molybdenum-bearing steels showed the highest corrosion resistance in a sulphuric acid solution, measured by using the EIS.     

Einflüsse des Phosphors auf die Interkristalline Spannungsrißkorrosion von Kohlenstoffstählen

Influence of phosphorus on the intergranular stress corrosion cracking of carbon steels The effects of phosphorus on the intergranular stress corrosion cracking were studied for steels with 0.15% C and 0.4 or 2% Mn, the phosphorus contents were 0.003, 0.03 and 0.05% P. Constant strain rate tests were conducted at constant potentials in 55% Ca(NO₃)₂ at 75°C, in 5 N NH₄NO₃ at 75°C, and in 33% NaOH at 120°C. The strain rate was 10^?6/s. Different grain boundary concentrations of phosphorus were established by varying the annealing time at 500°C, they were determined by fracturing the samples in UHV and analyzing intergranular fracture faces by Auger-electron spectroscopy. In the nitrate solutions the toughness, i.e. resistance against stress corrosion cracking of the steels is somewhat decreased with increasing P-content within the range of the corrosion potential, that is ?300 mV_H to ?50 mV_H in NH₄NO₃ and ?80 mV_H to ?50 mV_H in Ca(NO₃)₂. It is shown that this effect is caused by phosphorus segregated at the grain boundaries. At potentials above ?50 mV_H the relative toughness is very low for all tested steels, also without stress intergranular attack is observed. In NaOH the steels are most sensitive against stress corrosion cracking at ?700 mV_H, here the phosphorus content or segregation shows no effect. At potentials varied in both directions the toughness increases and a somewhat negative effect of phosphorus becomes visible. In all tested electrolytes the effect of phosphorus is restricted to a small potential range. There is no effect of phosphorus in the range of the minimal toughness, thus the steels are sensitive anyway, with and without phosphorus segregation. Therefore low phosphorus contents in carbon steels cannot guarantee resistance against stress corrosion, most decisive are the potential and the electrolyte.     

Stress corrosion cracking of bainite 0.3C‐1Cr‐1Mn‐1Si‐1Ni type steel in acid rain simulated solution

The effect of carbon content, heat treatment and surface treatment of high strength aircraft 0.3C‐1Cr‐1Mn‐1Si‐1Ni type steel on susceptibility to pitting corrosion, on hydrogen transport and on hydrogen embrittlement has been studied in acid rain simulated solution. Under open circuit conditions, susceptibility to stress corrosion cracking is associated with susceptibility to pitting corrosion and decreases with decreasing carbon content and at application of shot peening. Susceptibility to hydrogen embrittlement governed by hydrogen trapping is promoted by carbon segregation at boundaries of bainite laths and parent austenite grains. Decrease in the carbon content caused the decrease in hydrogen trapping resulting in increasing resistance to hydrogen embrittlement. Shot peening increased the trapping efficiency within the deformed layer, but the presence of the shot peened layer decreased the hydrogen flux entering the core and thus decreased the susceptibility of the core to hydrogen embrittlement. The shot peened low carbon steel, exhibiting mechanical properties required by the appropriate Standard and high resistance to stress corrosion cracking may be considered to replace the standard 0.3C‐1Cr‐1Mn‐1Si‐1Ni steel for the aircraft parts.     

Replacement of Nitrided 33CrMoV Steel with ESR Hot Work Tool Steels for Motorsport Applications: Microstructural and Fatigue Characterization

In this work tensile strength, fatigue resistance and fracture toughness of two electroslag remelted (ESR) tool steels and of 33CrMoV12 ESR steel (both in quenched and tempered condition, as well as nitrided condition) were evaluated. The role of hardness, residual stresses and inclusion sizes on the fatigue behavior was investigated. Tool steels have a tensile strength between about 1900 and 2300 MPa, fracture toughness between 35 and 33 MPa√m, while fatigue strength ranges between 725 and 992 MPa. The tensile strength and fracture toughness of the 33CrMoV12 ESR are, respectively, 1365 MPa and about 150 MPa√m. Nitriding induces a significant increase in fatigue strength from 560 to 980 MPa. These results highlight that appropriate ESR tool steels could replace nitrided steels.     

Grain Boundary Segregation Behavior in 2.25Cr-1Mo Steel During Reversible Temper Embrittlement

Low alloy steels serving for a long time at high temperature, e.g., around 500 °C, are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in fracture morphology of low alloy steels. This article describes the segregation behavior of various elements in 2.25Cr-1Mo pressure vessel steel investigated by AES, FEG-STEM, SEM, and EDS analyses. As confirmed by AES and FEG-STEM, phosphorus is found to be the main embrittling element for isothermal embrittlement. Sulfur and Mo segregation is only evident after longer embrittlement times. In the step-cooling embrittlement, phosphorus is still found to be the main embrittling element, but heavy segregation of sulfur in some isolated intergranular facets was also observed. For P segregation, a Mo-C-P interaction is observed, while sulfur segregation is attributed to site competition between sulfur and carbon atoms.