Surface Fatigue Strength Research Articles

Surface Analysis after Shot Peening Processing of ENAW 7075 Aluminium

Shot peening has been a very important and well-known surface cold-hardening process applicable to various machine parts and tool surfaces for quite a long time since it improves fatigue strength and corrosion resistance of stressed surfaces. Hardening by shot peening provides, because of plastic deformation, an increase in hardness and induces compressive residual stresses in the thin surface layer, which reduces material fatigue and prevents the generation and propagation of cracks. The present paper reports of results of shot peening hardening of specimens made of 7075-T651 aluminium. They were surface-hardened by steel shots M170 with Almen hardening intensity of 8A and 12A respectively and by glass beads GP165 with Almen hardening intensity of 12A. The steel shots and glass beads had the same diameter but differed considerably in hardness and density. The surface condition after shot peening processing was assessed in terms of surface integrity, which establishes a relation between processing conditions and roughness state and hardened-layer depth supported by an analysis of residual-stress variations.

White layer formation in hard turning of H13 tool steel at high cutting speeds using CBN tooling

White layers formed during machining have negative effects on surface finish and fatigue strength of products. The white layer is generally a hard phase and leads to the surface becoming brittle causing crack permeation and product failure. This is a major concern with respect to service performance especially in the aerospace and automotive industries. Numerous authors have investigated the formation of white layer under different manufacturing processes. In turning, it was suggested that the white layer structure is a martensitic phase whose formation is correlated to tool wear. Past studies have tended to concentrate on the formation of white layers at conventional cutting speeds, but never examined the formation at high cutting speeds. This paper reports on an investigation of white layer formation for wide range of cutting speeds in hard turning of 54-56 HRC H13 tool steel. The specimens were analysed using a micro hardness tester, SEM with EDAX software and Electron Micro-Probe. In addition tool wear and workpiece temperature were studied. The machined surface showed an increase in hardness with respect to the bulk material. Compositional gradients were noted for the white layer in terms of depletion of the elements iron and chromium coupled with an enrichment of carbon and oxygen content. The results showed that despite tool wear increasing with cutting speed, white layer depth and hardness actually reduced. This finding suggests that there may not be a direct relationship between white layer formation and wear, the correlation maybe linked to wear mode.

Effects of working parameters on surface finish in ball-burnishing of hardened steels

Burnishing is a chipless finishing method, which employs a rolling tool, pressed against the workpiece, in order to achieve plastic deformation of the surface layer. Recent developments made possible burnishing of heat-treated steel components up to 65 HRC. Features of burnishing include a good roughness (comparable to grinding), as well as improvement of mechanical characteristics of the surface (fatigue strength, corrosion resistance, and bearing ratio), due to implementation of compressive stresses into the surface layer. This paper will present influences of certain burnishing parameters upon roughness, for a hardened steel component (64 HRC).

Axial fatigue of a gas‐nitrided quenched and tempered AISI 4140 steel: effect of nitriding depth

ABSTRACTFatigue testing under fully reversed axial loading (R=−1) and zero‐to‐tension axial loading (R= 0) was carried out on AISI 4140 gas‐nitrided smooth specimens. Three different treatment durations were investigated in order to assess the effect of nitriding depth on fatigue strength in high cycle fatigue. Complete specimens characterization, i.e., hardness and residual stresses profiles (including measurement of stabilized residual stresses) as well as metallographic and fractographic observations, was achieved to analyse fatigue behaviour. Fatigue of the nitrided steel is a competition between a surface crack growing in a compressive residual stress field and an internal crack or ‘fish‐eye’ crack growing in vacuum. Fatigue life increases with nitriding depth until surface cracking is slow enough for failure to occur from an internal crack. Unlike bending, in axial fatigue ‘fish‐eye’ cracks can initiate anywhere in the core volume under uniform stress. In these conditions, axial fatigue performance is lower than that obtained under bending and nitriding depth may have no more influence. In order to interpret the results, special attention was given to the effects of compressive residual stresses on the surface short crack growth (closure effect) as well as the effects of internal defect size on internal fatigue lives. A superimposed tensile mean stress reduces the internal fatigue strength of nitrided steel more than the surface fatigue strength of the base metal. Both cracking mechanisms are not equally sensitive to mean stress.

Surface Temperature Calculation and its Application to Surface Fatigue Strength Evaluation

In recent years, power transmission gears for vehicles run at higher speeds and loads and are accompanied by very high surface temperatures. Under such a high surface temperature condition, the surface strength cannot be evaluated correctly only by the use of Hertzian stress. This research attempts to introduce a new surface strength evaluation method considering the surface temperature rise. First, the surface temperatures of rollers under different rolling and sliding conditions are measured using a thermocouple. The effects of load P, mean tangential velocity Vm and sliding velocity Vs on surface temperature are investigated and analyzed. Then, an experimental equation is presented which shows the linear relationship between surface temperature and GRP0.86Vs1.31Vm−0.83 value. Based on the comparisons between calculated and measured tooth surface temperatures, this equation is confirmed applicable to gear tooth surface temperature evaluation as well. A surface temperature index is proposed for surface strength evaluation. The relationship between the surface temperature index and the number of load cycles of the rollers is investigated. The results indicate the possibility to evaluate the surface strength based on the surface temperature.

748 軟窒化処理材におけるショットピーニング効果に関する研究

The authors investigated the refined surface characteristics and fatigue strength of Nitrocarburized (N) materials treated by Electro-polishing (EP), Double Shot Peening (DSP) and Stress Double Shot Peening (SDSP). The results obtained are summarized as follows: (1) Increased hardness with the diffusion layer between the surface and depth of 400 μm by DSP. (2) Remarkably high compressive residual stresses by DSP: Surface residual stres =-701 MPa, Maximum compressive residual stress=-733 MPa. (3) On the loading condition of stress ratio (R)=0, the fatigure limits of DSP and SDSP materials were 33% and 58% higher than that of N material, respectively. However, on the condition of R=-1, these were 28% and 39% higher than that of N material, respectively. (4) Same fatigue limit of both DSP and EP+DSP materials. (5) On the condition of R=0, the fatigue limit becomes high according to the lager compressive residual stress. However, on the condition of R=-1, the above relation is not necessarily valid to yield of the material by the compressive loading stress.

K-0711 ボールバニシング加工によるS35C材の表面改質と疲労強度改善(S07-3 疲労信頼性の向上(熱処理,加工))(S07 表面改質とその機能特性評価)

K-0711 ボールバニシング加工によるS35C材の表面改質と疲労強度改善(S07-3 疲労信頼性の向上(熱処理,加工))(S07 表面改質とその機能特性評価)

Evaluation of Surface Fatigue Strength Based on Surface Temperature. (Surface Temperature Calculation for Rolling-Sliding Contact).

Surface temperature is considered to be an integrated index that is dependent on not only the load and the dimensions at the contact point but also the sliding velocity, rolling velocity, surface roughness, and lubrication conditions. Therefore, the surface durability of rollers and gears can be evaluated more exactly and simply by the use of surface temperature rather than Hertzian stress. In this research, surface temperatures of rollers under different rolling and sliding conditions are measured using a thermocouple. The effects of load P, mean velocity Vm and sliding velocity Vs on surface temperature are clarified. An experimental formula, which expresses the linear relationship between surface temperature and the P0.86Vs1.31Vm-0.83 value, is used to determine surface temperature. By comparing calculated and measured temperature on the tooth surface of a gear, this formula is confirmed to be applicable for gear tooth surface temperature calculation.

212 コニカルギヤの歯面疲労強度に関する研究(OS1-4 伝動装置の基礎と応用)

212 コニカルギヤの歯面疲労強度に関する研究(OS1-4 伝動装置の基礎と応用)

Further improvements in some properties of shot peened components using the burnishing process

Shot peening and burnishing are considered to be cold working processes, where both processes are used to improve some metallic surface properties, due to the plastic deformation induced into these surfaces. The first process causes an increase in the roughness, while the other, in opposition to the first process, improves the smoothness of metallic surfaces. The present work is an attempt to apply the burnishing process onto shot peened components, in order to decrease the roughness of the metallic surface caused by the shot peening process. From the results obtained it was found that the burnishing process not only improves the surface finish, but it also introduces further improvements in surface hardness, fatigue strength and corrosion resistance for the two non-ferrous metals considered in this work, namely aluminium and brass. Accordingly, it can be suggested that burnishing of shot peened components may be used with benefits in industry for further improvements in the properties of metallic surfaces.

Efficiency of process methods for improving the surface quality and fatigue strength of gas-turbine engine disks of the KhN73MBTYu-VD alloy

The results of the present study suggest that a fatigue limit of the crown interslot projections of gas-turbine engine disks made of the KhN73MBTYu-VD alloy can be raised by subjecting the surface layer of the disk slots to ultrasonic hardening with balls. We attribute this to a removal of abrasive grits left by previous machining, to a decrease in depletion of the surface in alloying elements, and a simultaneous improvement in uniformity of the stress-strain state of various sections of the surface of a slot.

Influence of laser hardening on mechanical properties of a low alloy steel

An investigation was carried out to examine the influence of laser hardening on the mechanical properties of 300M steel. The mechanical properties(including high cycle fatigue strength) of 300M steel surfaces hardened by several conventional techniques such as shot peening, carbonitriding, and induction hardening have been compared with the mechanical properties of CO2 laser hardened specimens. The effects of such sUlface coatings as manganese phosphate and colloidal graphite on the laser hardening were also examined. The influence of all these hardening methods on the nature and magnitude of the resulting residual stresses was studied. The results of the present investigation indicate that the induction hardening produces the largest improvement in high cycle fatigue strength. Laser hardened specimens with manganese phosphate coating also showed significant improvement in high cycle fatigue strength over untreated specimens. In contrast, the high cycle fatigue strength of the graphite coated laser treated specimens was lower than that of untreated specimens. Test results also show that laser hardening produced surface hardness comparable to that achieved by induction hardening. However, the ductility of the laser hardened specimens (both manganese phosphate and graphite coats) was significantly lower than that of the induction hardened specimens. The major problem appears to be the formation of a diffusion bonded phosphate layer on the interface of manganese phosphate coated specimens.The lower fatigue strength and ductility of the laser treated graphite coated specimens appear to be a result of surface melting. Induction hardening introduced significant amounts of compressive residual stresses into the surface of these specimens.

Analysis of the compatibility of plasma-nitrided steels with ceramic coatings deposited by the ion-plating technique

Currently there is a trend towards deposition of thin films such as TiN or CrN on plasma-nitrided steels. This combination should enhance the surface properties and fatigue strength of the treated parts, but many problems have yet to be overcome and, in particular, the formation of a thin soft layer of ferrite located at the coating and γ' or ε compound layer interface. We present results on the formation of this interfacial film complementary to the results already published. We investigated the influence of the coating conditions on the structural stability of nitrided layers and found that the compatibility of these two treatments depends not only on the thermal sequence of the coating process but also on the ion bombardment during the etching step of the coating process.

Plasma surface engineering of low alloy steel

The surface of low alloy steel (En40B) has been engineered in the plasma of a glow discharge via plasma nitriding and ion plating of titanium nitride (TiN) coatings on the nitrided substrates with the purpose of enhancing the surface properties and fatigue strength. The nitriding response of the steel has been accessed by the evaluation of phase composition, layer thickness, hardness profile, residual stresses and nitrogen and carbon distributions. The wear and fatigue characteristics of the plasma-nitrided steel have been investigated and simple models have been developed to describe the influence of such properties as depth and strength of the nitrided case on the fatigue limit and load-bearing capacity of the nitrided steel. In order to further improve the tribological properties and load-bearing capacity of the low alloy steel, a duplex plasma surface-engineering technique has been developed. This is achieved by plasma nitriding the steel first so as to produce a thick, strong subsurface and then depositing a thin, hard and wear-resistant TiN coating on the nitrided substrate by ion plating. Dry-sliding wear tests demonstrated that the duplex-treated steel, i.e. the TiN coating-nitrided steel composite, not only exhibited enhanced wear resistance over the as-nitrided steel (by a factor of 2–8) but also had much higher load-bearing capacity than the TiN coating on unnitrided steel. Optimization of the coating-substrate combination can be achieved by correct control of the plasma-nitriding, surface preparation and ion-plating processes.

The influence of coating processes and process parameters on surface erosion resistance and substrate fatigue strength

This paper focuses on the surface erosion resistance and substrate fatigue strength for selected coating processes which have been shown to give the best erosion resistance and fatigue behavior in prior investigations. An iron boride coating applied using optimum process parameters shows 3 times more erosion resistance than an iron boride coating applied using unsuitable process parameters. The erosion resistance of a hard carbide material is dependent on the coating process used to apply this hard material, the final chromium carbide phase and the coating quality. The process parameters of a boride coating can decrease the substrate fatigue strength by between 10% and 58% of that of an uncoated blading steel depending on the process parameters used. This investigation shows that the coating process parameters greatly influence the surface erosion rate and fatigue strength of steel substrates.

ＣＢＮホイール加工による表面品位及び疲労強度の向上

The CBN wheel has been noted for its advantageous properties of long life expectancy and exceptional grinding efficiency and accuracy. In this paper, the authors look at the CBN wheel from the viewpoint of the quality of workpieces processed by the CBN wheel to compare the work surface integrity and fatigue strength obtained by the CBN wheel with those by a conventional SA wheel. The CBN wheel can improve the quality and strength of work materials. This suggests the significant possibilities of the CBN wheel for reducing the size and improving the efficiency of products, if it replaces the conventional wheels; namely, the processing by the CBN wheel even has an impact on product design.

SURFACE IRREGULARITY AND FATIGUE STRENGTH OF GUSSET JOINT UNDER VARIOUS CORROSIVE ENVIRONMENTS

SURFACE IRREGULARITY AND FATIGUE STRENGTH OF GUSSET JOINT UNDER VARIOUS CORROSIVE ENVIRONMENTS

Ｓ３５Ｃ鋼はめあい軸の疲労強度に及ぼすロール加工圧力の影響

By using the specimens of press-fitted part of axles which were cold surface-rolled under various rolling pressures, the relations among its mechanical properties, surface residual stress and fatigue strength, and the change in surface residual stress during the process of fatigue have been studied.The results obtained are as follows.(1) The fatigue strength increased linearly with the increase in rolling pressure. For the specimen of mean Hertz pressure 2.83×103MPa, the fatigue strength was 277MPa and its increasing rate relative to that of the normalized specimen was 216%.(2) The fatigue strength increased as the work-hardening volume and surface compressive residual stress increased, and linear correlations existed between the fatigue strength and these quantities.(3) The surface compressive residual stress decreased with stress cycle. It decreased linearly to the logarithm of number of stress cycles at the outside of the fitted part, and decreased curvilinearly at the inside of the fitted part. The decreasing rate at the inside of the fitted part was over two times faster than that at the outside. As cyclic stress increased, the decrease of surface compressive residual stress became more notable.

Effect of surface-plastic strain on the surface quality and fatigue strength of titanium ?-alloy

Effect of surface-plastic strain on the surface quality and fatigue strength of titanium ?-alloy

The significance of the hardness depth and the effect of notches on the fatigue strength of a tool steel for various rates of spark erosion

Spark-erosion has a marked effect on both surface hardness and fatigue strength of tool steels. This paper gives the relations between the hardness depth, the surface finish parameters and the decrease in the fatigue strength of a 90 MnV8 tool steel for various rates of spark-erosion. Both hardness and maximal depths proved to be of great significance regarding the decrease in the fatigue strength due to spark-erosion. To acquaint the designer with the effect of spark-eroded notches on the fatigue strength; a comparative study with ground notches is also presented. A “Process Factor” is introduced to assess the fatigue strength reduction factors of spark-eroded notches with various shapes.