Residual stress influence on fatigue lifetimes of electroplated AISI 4340 high strength steel

Residual stresses play an important role in the fatigue lives of structural engineering components. In the case of near surface tensile residual stresses, the initiation and propagation phases of fatigue process are accelerated; on the other hand, compressive residual stresses close to the surface may increase fatigue life. In both decorative and functional applications, chromium electroplating results in excellent wear and corrosion resistance. However, it is well known that it reduces the fatigue strength of a component. This is due to high tensile internal stresses and microcrack density. Efforts to improve hard chromium properties have increased in recent years. In this study, the effect of a nickel layer sulphamate process, as simple layer and interlayer, on fatigue strength of hard chromium electroplated AISI 4340 steel hardness – HRc 53, was analysed. The analysis was performed by rotating bending fatigue tests on AISI 4340 steel specimens with the following experimental groups: base material, hard chromium electroplated, sulphamate nickel electroplated, sulphamate nickel interlayer on hard chromium electroplated and electroless nickel interlayer on hard chromium electroplated. Results showed a decrease in fatigue strength in coated specimens and that both nickel plating interlayers were responsible for the increase in fatigue life of AISI 4340 chromium electroplated steel. The shot peening pre-treatment was efficient in reducing fatigue loss in the alternatives studied.     

Characteristics of hydrogen embrittlement,stress corrosion cracking and tempered martensite embrittlement in high-strength steels

Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diffraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel.     

Improvement in the fatigue strength of chromium electroplated AISI 4340 steel by shot peening

In landing gear, an important mechanical component for high responsible applications, wear and corrosion control is currently accomplished by chrome plating or hard anodising. However, some problems are associated with these operations. Experimental results have also shown that chrome‐plated specimens have fatigue strength lower than those of uncoated parts, attributed to high residual tensile stress and microcracks density contained into the coating. Under fatigue conditions these microcracks propagate and will cross the interface coating‐substrate and penetrate base metal without impediment. Shot peening is a surface process used to improve fatigue strength of metal components due to compressive residual stresses induced in the surface layers of the material, making the nucleation and propagation of fatigue cracks difficult. This investigation is concerned with analysis of the shot peening influence on the rotating bending fatigue strength of hard chromium electroplated AISI 4340 steel. Specimens were submitted to shot peening treatment with steel and ceramic shots and, in both cases, experimental results show increase in the fatigue life of AISI 4340 steel hard chromium electroplated, up to level of base metal without chromium. Peening using ceramic shot resulted in lower scatter in rotating bending fatigue data than steel shots.     

The crack tip strain field of AISI 4340 Part III Hydrogen influence

This paper studied the influence of hydrogen and water vapour environments on the plastic behaviour in the vicinity of the crack tip for AISI 4340. Hydrogen and water vapour (at a pressure of 15 Torr) significantly increased the crack tip opening displacement. The crack tip strain distribution in 15 Torr hydrogen was significantly different to that measured in vacuum. In the presence of sufficient hydrogen, the plastic zone was larger, was elongated in the direction of crack propagation and moreover there was significant creep. These observations support the hydrogen enhanced localised plasticity model for hydrogen embrittlement in this steel. The strain distribution in the presence of water vapour also suggests that SCC in AISI 4340 occurs via the hydrogen enhanced localised plasticity mechanism.     

Hydrogen embrittlement property of a 1700-MPa-class ultrahigh-strength tempered martensitic steel

The hydrogen embrittlement property of a prototype 1700-MPa-class ultrahigh-strength steel (NIMS17) containing hydrogen traps was evaluated using a slow strain rate test (SSRT) after cathodic hydrogen precharging, cyclic corrosion test (CCT) and atmospheric exposure. The hydrogen content in a fractured specimen was measured after SSRT by thermal desorption spectroscopy (TDS). The relationship between fracture stress and hydrogen content for the hydrogen-precharged specimens showed that the fracture stress of NIMS17 steel was higher, at a given hydrogen content, than that of conventional AISI 4135 steels with tensile strengths of 1300 and 1500 MPa. This suggests better resistance of NIMS17 steel to hydrogen embrittlement. However, hydrogen uptake to NIMS17 steel under CCT and atmospheric exposure decreased the fracture stress. This is because of the stronger hydrogen uptake to the steel containing hydrogen traps than to the AISI 4135 steels. Although NIMS17 steel has a higher strength level than AISI 4135 steel with a tensile strength of 1500 MPa, the decrease in fracture stress is similar between these steels.     

Hydrogen embrittlement property of a 1700-MPa-class ultrahigh-strength tempered martensitic steel

Abstract

The hydrogen embrittlement property of a prototype 1700-MPa-class ultrahigh-strength steel (NIMS17) containing hydrogen traps was evaluated using a slow strain rate test (SSRT) after cathodic hydrogen precharging, cyclic corrosion test (CCT) and atmospheric exposure. The hydrogen content in a fractured specimen was measured after SSRT by thermal desorption spectroscopy (TDS). The relationship between fracture stress and hydrogen content for the hydrogen-precharged specimens showed that the fracture stress of NIMS17 steel was higher, at a given hydrogen content, than that of conventional AISI 4135 steels with tensile strengths of 1300 and 1500 MPa. This suggests better resistance of NIMS17 steel to hydrogen embrittlement. However, hydrogen uptake to NIMS17 steel under CCT and atmospheric exposure decreased the fracture stress. This is because of the stronger hydrogen uptake to the steel containing hydrogen traps than to the AISI 4135 steels. Although NIMS17 steel has a higher strength level than AISI 4135 steel with a tensile strength of 1500 MPa, the decrease in fracture stress is similar between these steels.     

Effect of specimen size on fatigue crack growth rate in AISI 4340 steel

An investigation has been carried out to study the influence of specimen size parameters (thickness, with and aspect ratio) on fatigue crack growth rate. Compact tension specimens with a TL orientation, prepared from aircraft quality AISI 4340 steel and heat treated to a yield strength level of 1000 MPa, were used. All testing was done at a constant δK level. The investigation demonstrates that specimen thickness and width have no significant influence on fatigue crack growth rate for AISI 4340 steel. On the other hand, fatigue crack growth rate was found to increase marginally at high aspect ratios (a/W0.55). Paris constants C and m were also evaluated.     

3.5%NaCl腐蚀环境下2种航空铝合金材料疲劳性能试验研究

腐蚀环境下的疲劳性能是航空金属结构疲劳寿命设计的重要前提,为此,试验测定了2种航空铝合金材料（2E12-T3、7050-T7451）的光滑试样和缺口试样在干燥大气和3.5%NaCl腐蚀环境下的疲劳性能,在试验数据的基础上进行性能对比,并对试样断口进行扫描电镜（SEM）分析,研究了3.5%NaCl腐蚀环境与载荷联合作用对腐蚀疲劳性能的影响机理,研究结果表明:3.5%NaCl腐蚀环境对2种铝合金材料的疲劳性能均产生不利影响,且腐蚀与疲劳载荷的交互作用随着应力水平的降低而增强,疲劳性能下降更明显;与光滑试样相比,腐蚀环境对铝合金2E12-T3缺口试样疲劳性能的影响更大,但对铝合金7050-T7451缺口试样疲劳性能的影响却变小;在腐蚀环境下,裂纹尖端易发生电化学反应产生腐蚀产物和[H]离子,腐蚀产物的存在会阻碍裂纹闭合,同时,[H]离子导致裂纹尖端的氢脆效应,加快裂纹扩展,使疲劳性能降低。     

THE EFFECT OF ENVIRONMENT ON FATIGUE CRACK GROWTH BEHAVIOR OF 2021 ALUMINUM ALLOY

Abstract— The effects of dry hydrogen, moist air, distilled water and hydrazine environments on the fatigue crack propagation behavior of 2021 aluminum alloy have been investigated over a wide range of growth rates spanning about six orders of magnitude. Environmental interactions in the intermediate and near-threshold crack growth regions are shown to be associated with different fracture characteristics and mechanisms. Scanning Auger and X-ray photoelectron spectroscopic analysis of fracture surface corrosion deposits revealed that oxide induced crack closure phenomena, which considerably influence the near threshold corrosion fatigue behavior of low strength steels and some aluminum alloys, are not of importance for the present 2021-T6 aluminum alloy. The mechanistic aspects of environmentally influenced fatigue behavior of the alloy are discussed in the light of hydrogen embrittlement, chemical reactions and crack closure concepts.     

Atom probe tomographic observation of hydrogen trapping at carbides/ferrite interfaces for a high strength steel

A three-dimensional atom probe (3DAP) technique has been used to characterize the hydrogen distribution on carbides for a high strength AISI 4140 steel. Direct evidence of H atoms trapped at the carbide/ferrite interfaces has been revealed by 3DAP mapping. Hydrogen is mainly trapped on carbide/ferrite interfaces along the grain boundaries. Slow strain rate tensile (SSRT) testing shows that the AISI 4140 steel is highly sensitive to hydrogen embrittlement. The corresponding fractographic morphologies of hydrogen charged specimen exhibit brittle fracture feature. Combined with these results, it is proposed that the hydrogen trapping sites present in the grain boundaries are responsible for the hydrogen-induced intergranular fracture of AISI 4140. The direct observation of hydrogen distribution contributes to a better understanding of the mechanism of hydrogen embrittlement.     

Fatigue behaviour of AISI 304 steel to AISI 4340 steel welded by friction welding

In the presented study, The weldability of AISI 304 austenitic stainless steel to AISI 4340 steel joined by friction welding in different rotational speeds and fatigue behaviour of friction-welded samples were investigated. Tension tests were applied to welded parts to obtain the strength of the joints. The welding zones were examined by scanning electron microscopy (SEM) and analyzed by energy dispersive spectroscopy (EDS). The Vıckers microhardness distributions in welding zone were determined. Fatigue tests were performed using a rotational bending fatigue test machine and the fatigue strength has been analysed drawing S-N curves and critically observing fatigue fracture surfaces of the tested samples. The experimental results indicate that mechanical properties and microstructural features are affected significantly by rotation speed and the fatigue strength of friction-welded samples decrease due to chromium carbide precipitation in welding zone with increasing rotation speed in choosen conditions.     

Investigation of the mechanical properties of friction-welded joints between AISI 304L and AISI 4340 steel as a function rotational speed

In this study, standard AISI 304L austenitic stainless steel and AISI 4340 steel couple were welded by friction welding process using five different rotational speeds. The joining performances of AISI 304L/AISI 4340 friction-welded joints were studied and the influences of rotational speed on the microstructure and mechanical properties of the welded joints were also estimated. The microstructural properties of heat affected zone (HAZ) were examined by scanning electron microscopy (SEM). The microhardness across the interface perpendicular to the interface was measured and the strength of the joints was determined with tensile tests. The experimental results indicate that the tensile strength of friction-welded 304L/4340 components were markedly affected by joining rotational speed selected.     

Sulfide stress cracking resistance of API-X100 high strength low alloy steel

Sulfide Stress Cracking (SSC) of API-X100 high strength low alloy steel was evaluated in NACE solution “A” at room temperature. The corrosion rate, utilizing electrochemical polarization techniques, in the solution was 97 mpy. Proof ring testing, per NACE TM-0177, generated an SSC threshold stress value of 46% of yield strength. SSC susceptibility was caused by the high corrosion rate which formed corrosion pits that acted as crack initiation sites on the metal surface and provided more hydrogen to migrate into the steel. In addition, the X100 inhomogeneous microstructure provided a high density of hydrogen traps which promoted hydrogen embrittlement.     

Notch Severity Effect on Hydrogen Embrittlement of Type 4340 Steel

Hydrogen embrittlement of Type 4340 steel has been investigated by straining round-notchedspecimens in 105 Pa hydrogen atmosphere at a constent cross-head spead of 1.4x 10-4 mm/s.The circumferentially notched specimens exhibited a significant embrittlement when their me-chanical behaviour in hydrogen was compared with that in air. Although the effect of notch depthon fracture strength in air is negligible, an increase in the depth of notch increase susceptibility toembrittlement when testing in gaseous hydrogen. However, analysis of the effects is complicatedby the facts that (i) the specimens show some degree of notch severity even when strained inair and (ii) the behaviour is complicated by the localised plastic deformation that may occur forrelatively shatlow notches. Such effects are eliminated at high stress concentration factors, sothere is a systematic loss in fracture stress in hydrogen as the notch sensitivity increases fromK=2.6 to 5.7 (where a 87% reduction of fracture stress occurs) but a relatively stable value isthen reached even for very severe notching bj fatigue pre-cracking. Whether or not the effectis due to increasing concentration of hydrogen in the triaxial stress region ahead of the notch,there is no doubt that increasing the stress concentration makes hydrogen more effective as anembrittlement agent.     

Study of the effect of heat treatment on hydrogen embrittlement of AISI 4340 steel

Delayed failure tests were performed on fully-quenched AISI 4340 steel tempered at 500 and 700° C, subjected to sustained tensile loads and cathodically charged with hydrogen (current density 10 mA cm^–2) in an aqueous solution of 0.1 N sulphuric acid. The aim was to study the effect of the microstructure on the behaviour of steel towards the embrittling action of hydrogen. The tests were carried out with two different research techniques in order to highlight this behaviour better. This was done with a view to making the use of the steel safer. With the methods used the results obtained, although substantially different from each other, showed that thermodynamically more stable structures are less sensitive to the phenomenon of hydrogen embrittlement.     

CORROSION FATIGUE OF STEEL IN VARIOUS AQUEOUS ENVIRONMENTS

Abstract— Corrosion fatigue of three steels namely: AISI 1018, AISI 4340 and 17–7 PH stainless steel are investigated in various environments. Testing mediums chosen are natural seawater, polluted sea water from an industrial site, drinking tap water produced by desalination process, 4% sodium chloride tap water solution and a laboratory air environment. Results obtained for the three steels are consistent in pointing out that pollutants in seawater do not contribute to an acceleration of corrosion fatigue. The aggressiveness of drinking water is attributed to its chemical instability, low alkalinity and bicarbonate content.     

Inhibitors for protecting steel against hydrogen embrittlement in acid media

The effect of certain acid corrosion inhibitors on the ductility of steel 45 was studied on wire specimens stressed in torsion in an aqueous sulfuric acid solution. It was shown that the best protection against hydrogen embrittlement of steel is offered by inhibitors AGMIB and APB.     

爆轰处理对20CrMo 钢氢脆敏感性的影响

本文研究了爆轰处理对不同组织和强度水平的20CrMo 钢氢脆敏感性的影响。结果发现爆轰处理加高温时效可显著降低钢的氢脆敏感性。对屈服强度分别为830MPa 和620MPa 的均相珠光体组织、爆轰处理对其氢脆敏感性没有或略有影响。爆轰处理显著增加了屈服强度为680MPa,组织为铁素体加珠光体材料的氢脆敏感性系数,同时显著降低它对不可逆氢脆的敏感性系数。     

Causes of damages to fan blades of the generator of a steam turbine

It is shown that the fracture of blades of the fan of a TGV-200 generator is induced by corrosion fatigue intensified by residual hydrogen accumulated in steel in the process of electroplating of a cadmium coating on the surface of blades. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 109–112, May–June, 1997.     

Analysis of Influence of Aqueous Mediums on Cyclic Crack Resistance of Steels

Three variants of physical models of crack growth during corrosion fatigue destruction of steel are proposed: energy model, model of hydrogen embrittlement, and model of anodic dissolution of metal in crack tip. It is mentioned that the anodic model is more preferable for quantitative analysis. Using variant calculations, good agreement of this model with experimental results is demonstrated. The dominating role of local anodic dissolution is revealed as the main mechanism activating fatigue destruction of moderate strength carbon and low alloy steels in an aqueous corrosion medium.