Effect of surface polishing on SCC susceptibility of sensitised type 304 stainless steel

Abstract

The effect of surface polishing on the stress corrosion cracking (SCC) susceptibility of sensitised SUS 304 stainless steel was studied by using the well controlled surface polishing method which has been developed for the surface preparation of the semiconductor material. The SCC susceptibility of the polished specimen was examined by using the slow strain rate testing method, at which the specimen was kept at a constant potential in 0·1M Na₂SO₄ solution kept at 368 K. The most effective suppression effect on the SCC susceptibility was found in the case of surface polishing using alumina slurry, followed by water soluble fullerene and diamond slurry, compared with the mechanical polishing with #0/6 paper. Surface roughness and pitting potential of polished surface were compared with the SCC susceptibility of polished sensitised SUS 304 stainless steel. It was found that the SCC susceptibility was decided mainly by the roughness factor rather than pitting resistance. Experimental evidences suggest that the SCC susceptibility was decided by the crack initiation being controlled by surface roughness.     

Prüfung auf Spannungsrißkorrosion

Stress corrosion testing The authors present a survey of the different methods used in stress corrosion testing of materials of construction. After weighing the advantages and shortcomings of the individual methods the requirements are explained which should be met by an optimum method for plant use. By using screw-type springs as charging elements these requirements can be met to a great extent. Some examples are used to illustrate the progress of elongation over the test duration as well as the influence of aggressive medium, surface treatment and specimen shape on specimen life under stress corrosion conditions.     

Rapid Flow Stress Characterization of Steel

This rapid flow stress characterization concept involves rapid heating to an initial test temperature, T ₁, followed by loading and short-time stress relaxation measurement, followed by heating to a higher temperature, T ₂, followed by loading and short-time stress relaxation measurement, followed by heating to T ₃, and so on. This test sequence can generate stress-strain rate-temperature data over a wide spectrum, with a single specimen. The principal advantage of this method of flow stress characterization is its short-time format. The cost of specimen preparation is modest, as well. Beyond this, the test methodology provides very accurate temperature control. It also tests a given metallurgical structure, with minimal complications from structural evolution due to plastic deformation. This study has demonstrated the feasibility of this method on plain carbon and austenitic stainless steels, using a Gleeble testing machine. This includes demonstrated consistency between single-test-per-specimen data and data derived from sequential testing on a single specimen, as well as consistency with conventionally developed flow stress data.     

Einflüsse bei der Prüfung der Spannungsrißkorrosion

Influence factors in the testing of stress corrosion cracking The results of the tests of stress corrosion cracking of austenitic stainless steels in MgCl₂ solution are significantly dependent on the type and method of the test. The testing method used in the Research Institute of the Schoeller-Bleckmann Steelworks Ltd. is discussed, and attention is drawn to the influence of a number of test conditions such as the preparation of the MgCl₂ solution, the preheating of the test vessel with water, the surface condition of the specimen, the diameter of the specimen, the covering of its shaft, the time at which the load is applied, and the advance potential applied. The correlation of the service life with stress, concentration and temperature is used as an example in order to show that the test method is apt to affect not only we service life but also the extent and, type of these correlations. It is therefore only in conjunction with the exact test conditions that the test results can be regarded as fully significant.     

Fatigue damage study of cold metal transition fusion-brazed aluminium/steel dissimilar joints

ABSTRACT

An aluminium/steel CMT-welded joint is mainly composed of fusion zone (FZ), interfacial intermetallic compound (IMC) zone, and heat-affected zone (HAZ). IMC and FZ are the weakest link in the welded joint. The fatigue fracture indicates that the lattice distortion caused by the lattice mismatch of Fe₂Al₅ and Fe is more likely to engender dislocation slipping and cracks easily nucleate during the fatigue testing. The hysteresis loop is shifted to the right, which indicating that the specimen experiences a ratcheting effect. In the high-stress condition, the cyclic hardening induced by the ratcheting strain is much stronger than that in the lower stress condition, which improves the fatigue life of the materials.     

Evolution of stress and morphology in thermal barrier coatings

Residual stress in the thermally grown oxide (TGO) in thermal barrier coatings (TBCs) was measured by photoluminescence piezospectroscopy (PLPS) and stress maps created to track local stress changes as a function of thermal cycling. The local stress images were observed to be correlated with morphological features on the metal surface that were purposely introduced during specimen preparation. Local stress relaxation and morphological evolution with thermal cycling were studied using the stress maps combined by post-mortem SEM examination. It was found that the morphology in the specimen having an initial polished surface was quite stable, while that in the specimen with a rough surface was unstable. The average residual stress in the specimen with the unstable morphology decreased with thermal cycling and it eventually failed along TGO/YSZ interface. The specimen with stable morphology maintained a high TGO stress throughout the thermal cycling process and failed along TGO/bond coat interface. The rough surface was also found to give rise to the formation of transition alumina (θ-Al₂O₃) in the TGO which was correlated with a reduced TGO stress.     

Evaluation of through thickness residual stress in multipass butt welded joint by using inherent strain

Abstract

The through thickness residual stress of an eight pass butt welded plate joint is evaluated using inherent strain analysis. The residual stress distribution is obtained in detail along the thickness direction from measurements using multiple strain gauges. The residual stresses agree with the results of the thermal elastic–plastic analysis as well as the values obtained by direct measurement of the specimen surface, which is not used in inherent strain analysis. These results indicate that both inherent strain analysis and thermal elastic–plastic analysis are effective in evaluating through thickness residual stress. Therefore, each analysis method should be chosen after considering the object to be evaluated and the characteristics to be analysed.     

Residual stress in a restrained specimen processed by post-weld ultrasonic impact treatment

ABSTRACT

A multipass thick welded specimen restrained by three saddle-shape plates welded to the bottom surface was prepared and treated using the post-weld ultrasonic impact treatment (UIT). The stresses in the specimen were measured and analysed. Results show that the as-welded stresses on the top surface of the restrained specimen do not drop quickly beyond the weld zone as compared with the typical welding stress distributions in thick unrestrained specimens. The UIT induces compressive stresses with a depth up to 2–4?mm and it has little effect on the as-welded stresses outside of the UIT-treated region. The welding of restraining plates causes a significant increase of the longitudinal stress within the local welding region.     

Welding of corrosion-resisting steels to titanium alloys

Summary

This paper describes a new method for measurement of the three‐dimensional residual stresses in fillet welds using inherent strains, which are considered to be a source of residual stresses. The new method implies first the inherent strains being determined from a few measured elastic strains with the aid of a proposed inherent strain distribution function, and second the residual stresses being calculated through an elastic analysis when the inherent strains are applied to welded joints.

The measurement method is called the TL_yL_z method when three types of T, L_y and L_z specimen are cut from the fillet weld, and the T method when only one specimen T is cut from the weld.

The TL_yL_z and T methods are respectively used to measure three‐dimensional residual stress distributions on the transverse sections of single‐pass and multipass T‐type fillet welds. The residual stresses measured by the TL_yL₂ and T methods show good agreement with the ones directly measured on the surfaces of welded joints, thus demonstrating the validity and practical applicability of the proposed method.     

Shear band evolution and hardness change in cold-rolled bulk metallic glasses

A systematic investigation of the shear band evolution and hardness change with deformation was performed on cold-rolled Zr_64.13Cu_15.75Ni_10.12Al₁₀ and Zr_46.5Cu₄₅Al₇Ti_1.5 bulk metallic glasses. It was found that primary shear bands reach saturation after 7% in thickness reduction and only primary shear bands exist below 20% in thickness reduction, based on statistical analyses of primary shear band spacing, angle and offset. Rolling creates more free volume, and deformation-induced residual stress distribution in a heavily rolled specimen relative to the as-cast specimen has been determined. Larger tensile residual stresses are generated on the side surface as compared with those on the top surface, while compressive residual stresses in the middle are induced. Such residual stresses strongly influence the hardness measured. It is also revealed that after stress relief, the hardness does not decrease considerably in heavily rolled/annealed specimens here as compared to as-cast/annealed specimens, probably due to low shear band density.     

Quantifying residual stresses in resistance spot welding of 6061-T6 aluminum alloy sheets via neutron diffraction measurements

Residual strains of resistance spot welded joints of 6061-T6 aluminum alloy sheets were measured in three different directions denoted as in-plane longitudinal (σ₁₁), in-plane transversal (σ₂₂), and normal (σ₃₃). The welding process parameters were established to meet or exceed MIL-W-6858D specifications (i.e., approximately 5.7 mm weld nugget and minimum shearing force of 3.8 kN per weld confirmed via quasi-static tensile testing). Electron backscatter diffraction (EBSD) and optical microscopy (OM) were performed to determine grain size and orientation. The residual stress measurements were taken at a series of points along the weld centerline at depths corresponding to the weld mid-plane and at both 1 mm below the top surface of the plate and 1 mm above bottom surface. The residual stresses were captured on the fusion zone (FZ), heat affected zone (HAZ) and base metal (BM) of the resistance spot welded joint. Neutron diffraction results show residual stresses in the weld are approximately 40% lower than yield strength of the parent material. The maximum variation in residual stresses occurs, as expected, in the vertical position of the specimen because of the orientation of electrode clamping forces that produce a non-uniform solidification pattern. Despite the high anisotropy of the welding nugget and surrounding area, a significant result is that σ₃₃ measured stress values are negligible in both the horizontal and vertical directions of the specimen. Consequently, microstructure–property relationships characterized here can indeed inform continuum material models for application in multiscale models.     

Zum Einfluß der mechanischen Belastung auf die interkristalline Spannungsrißkorrosion von unlegiertem Stahl in Carbonat/Bicarbonat-Lösung

Effect of mechanical stress on intergranular stress corrosion cracking of unalloyed steel in carbonate/bicarbonate solution Intergranular stress corrosion cracking of a carbon steel in 0.5 M NaHCO₃ + 0.5 M Na₂CO₃ solution at 70°C was examined by normal, interrupted and cyclic CERT testing. Within the critical potential range, the lengths of intergranular cracks increase with increasing strain rate, and decrease when the material is cold worked. The fracture time indicates that the crack velocity increases with increasing strain rate. This is also observed with tensile specimens subjected to interrupted and cyclic strain rates. Detailed information on crack propagation and threshold stress levels can be obtained by interrupted and cyclic CERT tests. Intergranular cracks arise if surface region of the material undergoes plastic deformation in excess of a definite amount. In the case of a 60°-notch specimen the plastic zone must be greater than 0.3 mm. For cyclically strained specimens the crack velocity can markedly decrease with time or can become even zero as a consequence of strain hardening. From this one can conclude that stress corrosion cracking can only occur if stress levels are very high.     

Tensile Test Specimens with a Circumferential Precrack for Evaluation of Interfacial Toughness of Thermal-Sprayed Coatings

A new testing procedure to evaluate the interfacial toughness of thermal-sprayed coatings has been developed. The newly designed test specimen is a modification of the pin test with an artificially introduced weak interface, which is expected to open up easily under tensile loading and act as a circumferential precrack along the interface between a coating and the substrate. This configuration makes it possible to calculate the stress intensity factor K _Int at the tip of the precrack, which can be expressed as , where σ₀ is the apparent average stress, a the crack length, R the specimen radius, and F _I the geometrical correction function. Finite-element analysis was carried out to calculate the correction function F _I for various values of a/R. In the experiments, the flat surface of a pin was grit-blasted and a ring-shaped area from the periphery was covered with carbon using a pencil and set into a mating dice. SUS316L stainless steel was plasma-sprayed onto the flat surface of the pin and the dice. Then, tensile load was applied to the pin to break the weak interface containing the carbon and finally the unmodified coating-substrate interface. The load required to pull out the pin was measured for various specimen parameters such as a and R. The results indicate that the adhesion of the tested coatings can be represented by interface toughness of 1.9 ± 0.1 MPa m^1/2. As a consequence, this testing procedure can be considered as a viable method to evaluate adhesion of a thermal-sprayed coating on a substrate.     

Apparatur zur Prüfung auf Spannungsrißkorrosion unter Druck

Apparatus for stress corrosion susceptibility testing under pressure Testing the susceptibility to intercrystalline stress corrosion in concentrated MgCl₂ solution is complicated by the following factors: high solidification temperature of the solution; the results obtained in concentrated solution cannot be extrapolated to the behaviour in diluted solutions, while an extrapolation is quite feasible in the case of diluted solutions at different temperatures (this case is more analogous to conditions encountered in practice). This method requires, however, the use of an apparatus for working under elevated pressure (up to 6 atm), which has been developed by the authors. A lever system enables loads up to 20 000 N to be applied; the elongation of the specimen is limited to 20 mm (specimen length 30 mm). The specimen potential is measured with a Haber-Luggin-Capillary and a thalamide reference electrode. The scattering of results is in conformity with the scattering which can be derived from other researchers' result (mean scattering factor = 1.5).     

Role of imposed potentials in threshold for caustic cracking susceptibility (KISCC): Investigations using circumferential notch tensile (CNT) testing

A fracture mechanics-based novel approach, i.e. circumferential notch tensile (CNT) testing has been employed for determination of threshold stress intensity factor for susceptibility of engineering materials to stress corrosion cracking (K_ISCC) using small specimens. Using CNT technique, K_ISCC of a carbon steel at an open circuit potential (E_corr) in 500 g L⁻¹ NaOH at 100 °C was determined to be 42.9 MPa m^1/2. In order to establish the application of the CNT technique in understanding the mechanistic aspects of caustic cracking as well as for developing guidelines for mitigation, tests have also been performed under the imposed electrochemical potentials. An imposed potential in the active–passive potential regime (E_a–p) caused an extremely rapid failure (than observed at E_corr) whereas, at an imposed potential in the passive region (E_p), the specimen did not fail even after relatively very long exposure time. The fractography of the CNT specimens tested at E_corr and E_a–p presented evidence of SCC. The study has established the use of experimental CNT testing as a simple, relatively fast and cost-advantageous approach for generating the K_ISCC data, which are also consistent with the electrochemical mechanism for caustic cracking.     

Oxidation of Rimming Steels in Carbon Dioxide Atmospheres*

Abstract

A study has been made of the kinetics and mechanisms of the oxidation resistance of rimming steels to carbon dioxide atmospheres over long periods of testing. The effects of moisture and carbon monoxide additions to the carbon dioxide, gas pressures up to 600 psig and temperatures from 350 to 550° have been investigated.

It is shown that protective oxide films are usually formed at a gas pressure of one atmosphere, but at high pressures and particularly with moisture and carbon monoxide in the gas, the oxide film may locally break down, giving rise to oxide excrescences which can grow in size and number to cover most or all of the surface of the specimen. The formation of excrescences causes the rate law to change from cubic or parabolic to linear over a transitional period. The reaction rate constants are dependent essentially on the temperature whilst moisture, carbon monoxide and gas pressure are mainly effective in promoting the initiation and development of excrescences.

Possible causes for the formation of excrescences, as well as the mechanism of the linear rate law during the post-breakaway period, are discussed.     

Low-cycle impact fatigue of SiCw/7475Al composite

Important uses in the future for metal-matrix composites are in aerospace, weaponry, and high-speed power plants in which the inertial force produced by great acceleration is a load of high strain rate. Therefore close attention is given to the mechanical behavior of a composite at high strain rates. This paper reports a study of the behavior and mechanisms of a SiC _w /7475 composite in low-cycle impact fatigue (LCIF). The LCIF and impact tension tests were conducted by using the push-pull impact fatigue apparatus developed by the authors, in which the loading assembly was actually a combination of a Hopkinson’s pressure bar and an extension bar. In the apparatus the trapezoidal stress wave loads were produced. The strain rates in specimens may reach 400 s ^-1 .The results show that for a SiC _w /7475 composite, the strain-rate effects on yield stress, ductility, cyclic hardening and softening, δ _e /2-N _f relation, and transition life were slight. In low-cycle impact fatigue the cracks often initiated within or near the SiC particles, which mingled in the composite. The SiC _w /7475 composite was found to be less ductile than its alloy matrix; in low-cycle fatigue brittleness appears. Therefore great attention must be given to the behavior of the composite when it is used as a structural material.     

Surface roughness and measurement with new contact methods

To obtain high-quality products requires, among others, testing of material properties and measurements of dimensions and errors of surface geometry. The development of surface roughness evaluation depends on the introduction of cheap and easily-applied measuring instruments. The most commonly used stylus-type devices are of complicated construction, expensive and require highly qualified quality control personnel or workers to operate. On the other hand, optical and pneumatic methods are, due to their sensitivity to disturbances and large measuring errors, by far less commonly used. Investigations are conducted in the Institute of Mechanical Technology at the Technical University of Warsaw on the development of methods to construct simple devices designed for measuring the surface roughness directly on the work-stand. Until now two contact methods have been developed with measuring elements movement restricted to the orthogonal direction in relation to the surface measured. The first one is an indirect method using the effect of interpenetration of rough surfaces and enables very simple measuring devices to be constructed, their measuring error reaching up to 20 per cent. The other one is a direct method of roughness levelling depth (R_p) measurement and enables measuring devices of much higher accuracy to be built.     

Fatigue damage monitoring of round shape specimen by acoustic microscopy

Acoustic microscope can detect microstructural features with high resolving power compared to other lower frequency acoustic techniques. Many non-destructive evaluations using acoustic microscope, several of them used in industrial fields, were investigated. However, acoustic measurement of curved surface structure was restricted to basic studies as conventional acoustic microscope system was designed for flat specimen. We have developed a modified acoustic microscope for curved surface specimen testing and have measured acoustic image of round bar specimen. In this study, using this system, velocities of round bar specimen during fatigue test are monitored using the V(z) curve method. Measurement errors depending on curved surface measurement were investigated and it was found that the acoustic velocity tends to increase as fatigue damage increases.     

Beitrag zur Spannungsrißkorrosion austenitischer Stähle

Contribution to the problem of stress corrosion cracking of austenitic steels It is shown on the basis of a stress corrosion cracking theories that the incubation phase is the factor determining the useful life of an element because in this phase protective layers are destroyed and crack nuclei are formed. Testing corrosion susceptibility in boiling magnesium chloride solution has the shortcoming that the solution is highly aggressive so that minor changes of the surface condition which may yield a considerable improvement of material behaviour in less aggressive conditions do not come to bear. Another shortcoming of MgC1₂ testing are divergent literature data concerning concentration so that there is also a scatter of boiling temperatures which has a pronounced influence on test results. Likewise an important role can be attributed to foreign ions and silicates from glas in the solution. It is more advantageous to use less concentrated solutions (up to 35%) because they enable clear current density potential curve to be obtained. An important factor concerning the test method is the previous investigation of specimens with a view to a residual stresses due to cold reduction. Under certain conditions the time to failure may be longer under cycling loads.