The effect of additional silicon on the corrosion and erosion-corrosion of low chromium steels

The addition of 1wt.%Si to commercial steel containing 2.5wt.%Cr significantly lowered its erosion-corrosion metal wastage rate at 650 °C. The improved erosion resistance was largely a result of a modified corrosion behavior. Static oxidation studies showed that the normal silicon content 2.5Cr steel developed a duplex, spalling prone, iron-rich scale at 650 °C. However, with the addition of silicon, the scale was almost 50 times thinner, and was chromium-rich instead. Metal wastage was significantly reduced for static and dynamic corrosion as well as for erosion-corrosion. The thin scale that formed on the additional silicon steels cracked and chipped off at a low metal loss rate compared with the thick scale that formed on the normal silicon steels that spalled off at a higher metal loss rate.     

Surface degradation of ductile metals in elevated temperature gas-particle streams

The mechanisms and rates of erosion and combined erosion-corrosion of 9Cr-1Mo steel (where the composition is in approximate weight per cent) and type 310 stainless steel at elevated temperatures were investigated to understand better the behavior of piping steels in fluidized bed combustor environments. Tests were performed in a partially inert gas atmosphere to study erosion behavior and in an air atmosphere to study combined erosion-corrosion behavior. It was determined that the erosion rate remained constant or decreased with increasing temperature in nitrogen until a temperature was reached at which the tensile strength started to decrease more rapidly with increasing test temparature. Above this temperature the erosion rate increased rapidly with temperature.In an erosion-corrosion environment, corrosion was the dominant mechanism at all test conditions. At higher temperatures and velocities the material loss mechanism changed from low loss rate chipping of the scale to high loss rate periodic spailing. The continuous scale formed on 9Cr-1Mo steel in air appeared to protect the metal surface, decreasing its loss rate in α = 30° tests compared with that of type 310 stainless steel tested in the same conditions in nitrogen where a continuous scale did not form.     

The high strain rate compression of 1023 and 4130 steels

Steel specimens of type 1023 and heat treated 4130 steel of hardnesses, between 257 and 450 HV10, were compressed at strain rates between 3 × 10³ and 3 × 10⁴ s⁻¹ using a modified Hopkinson bar. Both steels showed viscous type flow behaviour with a linear increase in flow stress with strain rate. For the 1023 steel there were two linear regions, one from 3 to 12 × 10³ s⁻¹ with a macroscopic viscosity of 4·7 kPa s and two from 14 to 27 × 10³ s⁻¹ with a viscosity of 0·7 kPa s. These results indicate some change in the controlling mechanism. For the 4130 steel the macroscopic viscosity decreased linearly with increasing hardness from 7·4 kPa s at 257 HV 10 to 2·2 kPa s at 450 HV 10. This was probably due to a decrease in the density of mobile dislocations with increased tempering temperatures. At lower hardness values (257 and 300 HV 10) and higher strain rates, a levelling off of the flow stress occurred but the data were too scattered to give a definite trend. The heat generated due to deformation of the 1023 steel was sufficient to produce steady state flow at strains above about 0·3. In the 4130 steel the thermo-mechanical properties were such that linear work softening was observed at a rate inversely proportional to the square root of the strain rate and independent of the hardness. This suggests that the work softening was a function of properties of the common ferrite matrix. The strain at which the work softening commenced decreased with increasing hardness, because of the more rapid heat generation at higher flow stresses. At higher hardness and higher strain rates 4130 steel failed by shear. Shear failure occurred at lower strain rates with increasing hardness and, for a given hardness, at lower strains with increasing strain rate. This behaviour was consistent with Recht's analysis of catastrophic shear failure.     

A comparison of the erosion rate of casing steels by sand-oil suspensions

The erosion rates (expressed as grams per square millimeter per minute) of three casing steels (P110, N80 and K55) and hot-rolled 1020 steel in quartz sand-oil suspensions, GFN 140, solids loading 0.5–4 wt.%, at speeds from 9.35 to 18.7 m s⁻¹ have been measured in a slurry pot tester. The velocity exponent was determined as 2.4 and the relationship of erosion rate to solids loading and particle size (for the particle size range 53–180 μm) determined. The erosion rates for all the steels were found to lie within a scatter band of ±15% and could not be related predictively with the measured tensile properties. Results are discussed in terms of the rate of particle impact on the eroding target and the need to control flow conditions through appropriate design to minimize erosion.     

Study of lubricated impact using optical interferometry

An experimental study of lubricated impacts between a steel ball and a flat glass surface has been performed. The experimental set-up consists of a Ø70 mm ball mounted on a pendulum which impacts onto a lubricated glass disc. The contact region is studied by means of optical interferometry using a monochromatic light source, a microscope and a high-speed video recording equipment.

The lubricants are of PAO type and the viscosity ranges from 27 to 2600 mm² s⁻¹ at the test temperature. The impact velocity is varied between 0.08 and 0.29 m s⁻¹.

A dimple occurs at the centre of the contact where the lubricant is trapped. The influence of viscosity and impact velocity on the dimple's depth and diameter is studied.     

Solid particle erosion of diamond coatings under non-normal impact angles

This paper describes an erosion study, which examines the effect of impact angle on the erosion behaviour of diamond coatings deposited on tungsten substrates by chemical vapour deposition (CVD). The coatings were 37–60 μm in thickness and were erosion tested using angular silica sand with a mean diameter of 194 μm at a particle velocity of 268 m s⁻¹. The impact angles used were 30, 45, 60 and 90°. The results show that the damage features, termed “pin-holes” are generated at all angles, though the number of impacts required for pin-hole initiation is significantly increased at lower angles. This work provides useful information in attempting to explain the mechanism by which damage is generated during the high velocity sand erosion of CVD diamond.     

Erosion of an aligned alumina-stainless steel composite

Steady-state erosion of a fully aligned composite consisting of a regular array of alumina rods embedded into a 304 stainless steel matrix has been investigated at room temperature using alumina erodents whose mean diameters varied from 37 to 390 μm. Impact angles varied between 15° and 90° with velocities in the range of 60–100 m s⁻¹. All experiments were performed with the axis of the alumina rods perpendicular to the erosion surface. Experiments under identical conditions were performed on bulk samples of 304 stainless steel and on the same type of alumina. The composite results can be described, to a reasonable approximation, using a model whose basis is that constrained steady-state erosion occurs such that the bulk stainless steel erosion rate determines the form of a stationary erosion-surface profile. The physical basis of the constraints are discussed in terms of the erosion mechanisms.     

High strain rate compression of titanium and some titanium alloys

A modified Hopkinson bar was used to compress specimens of commercially-pure titanium, IMI 125 and titanium alloys, AMS 4911B and AMS 4916B, at natural strain rates of between 3 × 10³ s⁻¹ and 3 × 10⁴ s⁻¹. All three materials deformed in a viscous manner with a linear increase of flow stress with strain rate and a macroscopic viscosity of 4·5 kPa s, 5·7 kPa s and 9·2 kPa s, respectively. At strain rates above about 1 × 10⁴ s⁻¹ there was a decrease in macroscopic viscosities to 0·9 kPa s and 2·5 kPa s for the titanium and 4911B alloy respectively, which is attributed to changes in the dislocation drag mechanisms. The results for the 4916B alloy at the higher strain rates were too scattered to give a definite trend. It is suggested that the increase in the macroscopic viscosity with alloying is due to an increase in the dislocation drag coefficient by solution hardening and to a reduction in the density of mobile dislocations by dislocation pinning.

It is also proposed that the susceptibility of these materials to catastrophic shear failure which occurs without prior linear work-softening is due to the higher propensity for titanium and titanium alloys to shear failure as a consequence of their thermo-mechanical properties. The decrease of strain at which shear occurred with increasing strength is in accord with this suggestion.     

Elevated temperature combined erosion-corrosion of steels

The effects of erodent particle size and elevated temperatures on the combined erosion-corrosion behavior of a series of chromium-containing steels were investigated in an oxidizing gas stream.It was determined that corrosion was the dominant mechanism at all test conditions for all alloys. Particle impacts enhanced the growth rate of multilayered iron oxide and Fe-Cr oxide spinel scales and markedly changed the morphology of the scale surface and the thickness of the various scale layers.     

The friction and wear properties of nylon

The effects of temperature on the friction of nylon are examined at various sliding velocities. Maximum friction values are obtained, but it is found that when specimens slide continuously at various velocities, the magnitude of the maximum friction varies with the velocity, while when specimens slide alternately at 10 and 0.1 mm s⁻¹, nearly the same maximum value is obtained. In the latter case, the displacement of the friction-velocity curves at various temperatures along the velocity axis can give a master curve while in the former case this is impossible.

The effects of temperature, velocity and load on the wear of nylon are also examined. Minimum values of the wear are obtained in all cases, whereas maximum values of the friction are obtained. The mechanism of such behaviour is discussed.     

Scale behavior in erosion-corrosion of low alloy steels

Low alloy steels containing some chromium and other alloying elements with regular silicon content, together with the same chromium content steels containing extra silicon, were tested at the velocities and temperatures which can occur in atmospheric fluidized-bed combustors. It was determined that the particular characteristics of the scale formed on the alloys contributed to the metal wastage that occurred. The low chromium content steels which contained extra silicon (1.4–1.5 wt.%) had markedly lower metal loss than regular silicon content steels had, even lower than that of higher chromium content, regular silicon content steels. The role of the extra silicon that increases the erosion-corrosion resistance of the steels is under investigation.     

Slurry wear properties of pump lining materials

In order to study the slurry wear properties of elastomeric lining materials for sand, sludge and sewage pumps, several tests were carried out under various test conditions with a slurry jet device and prototype pump. Preliminary tests were done with 13 materials including polymers, metals and ceramic coatings. Polyurethane, fluid elastomer and rubber showed good wear resistance compared with the hard metals, and were examined in detail. The test conditions were as follows: jet velocity v (8–25 m s⁻¹), impingement angle (10°−90°), mean diameter of silica sand d (42–415 μm), and sand concentration by weight c (0.1–7 wt.%). The slurry wear rates of the polyurethane and the fluid elastomer were maximum at the impingement angle of approximately 30°. The rubber showed almost the same wear rate regardless of impingement angle. The slurry were rate increased according to a power law of the jet velocity above a critical velocity and sand concentration. As the particle size increased above a critical size at the same weight concentration, the wear rate first increased but then decreased. The influence of impingement variables and particle variables observed using both test methods shows a similar tendency for each lining material, so the slurry wear loss can be described with a single empirical equation. Comparing the slurry wear resistance of three lining materials, the polyurethane showed the lowest resistance because it contained many pores which were produced during the lining process. The rubber showed the highest slurry wear resistance which is probably related to its high tear strength.     

Friction and wear properties of two types of copper — Graphite brushes under severe sliding conditions

High speed dry friction experiments using two copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities up to 230 m s⁻¹ and at current densities up to 526 A cm⁻². One brush was a commercial powder metallurgy (PM) specimen and the other was a graphite fiber-Cu/Sn matrix composite material. The composite brush was prepared by a proprietary process of liquid-metal infiltration and was run with the graphite fibers perpendicular to the rotor surface. The coefficient of friction was determined as a function of velocity, the wear rates were determined as a function of velocity and the voltage drop was determined as a function of velocity and current. The results show that the coefficient of friction exhibited by the PM brush is lower than that of the composite material at any velocity tested. The wear rates without current are much higher for the PM than for the composite brush, but they are of the same order when a current of 600 A is passed through. The voltage drop at the brush-rotor interface shows a similar variation with velocity for the two brushes, but the variation of the voltage drop as a function of current is different for the two specimens. The voltage drop increases almost linearly with increasing current for the PM brush. For the composite brush it exhibits a sharp increase up to about 50 A and then varies very little up to the maximum current of about 600 A. The damage done to the rotor surface in the case of heavy sparking is more pronounced with the PM brush than with the composite brush. It appears that the difference between the high temperature mechanical properties of the two types of brushes is responsible for their different behavior under severe sliding conditions.     

Wear mechanism of ceramic tools

Cutting tests were performed using ceramic cutting tools under continuous cutting conditions. The tests were carried out on AISI 1040 steel, with cutting speeds ranging from 5 to 11 m s⁻¹. The wear mechanism was investigated for both crater and flank. Alumina-toughened zirconia of submicron grain size showed the best wear resistance. Alumina with TiC, TiN and ZrO₂ inclusions exhibited a wear resistance a little lower than the above-mentioned materials. Low chemical stability seems to be the reason for the poor performances of the silicon carbide whiskers-reinforced alumina, silicon nitride and the tungsten carbide inserts.     

An examination of the electrochemical characteristics of two stainless steels (UNS S32654 and UNS S31603) under liquid-solid impingement

The erosion-corrosion resistance of high alloy stainless steel UNS S32654 and standard stainless steel UNS S31603 has been assessed under liquid-solid impingement conditions. The electrochemical characteristics of the two stainless steels have been examined via free corrosion potential measurements, anodic polarisation, linear polarisation and potentiostatic control in erosion-corrosion.It has been shown in this paper that high alloy stainless steel UNS S32654 exhibits better corrosion and erosion-corrosion performance than lower grade alloy UNS S31603. A general linear relationship between two electrochemical parameters (E_corr and R_p) has been shown in this study. A critical solid loading between 60 and 100 mg/l, at which there is a transition from corrosion to erosion-corrosion for the two stainless steels under different conditions, has been determined.     

Dynamic tensile characteristics of TRIP-type and DP-type steel sheets for an auto-body

This paper is concerned with the dynamic tensile characteristics of transformation-induced plasticity (TRIP)-type and dual phase (DP)-type steel sheets at intermediate strain rates ranging from 0.003 to 200 s⁻¹. The dynamic responses of TRIP600, TRIP800, DP600 and DP800 steel sheets are investigated with the evaluation of stress–strain curves, the strain rate sensitivity, the fracture elongation and the effect of pre-strain. The dynamic responses were acquired from dynamic tensile tests at the intermediate strain rates with a high-speed material testing machine developed. Experiments were carried out with specimens whose dimensions were carefully determined by finite element analyses and experiments to induce uniform deformation in the gauge section at the intermediate strain rates. The tensile tests provide stress–strain curves and the strain rate sensitivity. Experimental results show two important aspects for TRIP-type and DP-type steel sheets quantitatively: The flow stress increases as the strain rate increases and the fracture elongation and the formability of TRIP-type sheets are better than those of DP-type sheets at the intermediate strain rates. The pre-strain effect was also investigated for two types of metals at the intermediate strain rates. TRIP600 and DP600 steel specimens pre-stained by 5% and 10% were elongated at the strain rate of 0.003 s⁻¹ for quasi-static loading, and then tested at strain rates of 0.003, 1, 10 and 100 s⁻¹. The results demonstrate that the mechanical properties of TRIP600 and DP600 steels are noticeably influenced by the pre-strain when the strain rate is over 1 s⁻¹. The ultimate tensile strength as well as the yield stress increases due to the pre-strain.     

Control and evaluation of particle impact conditions in a sand erosion test facility

For the prediction of actual damage to plant component materials and for making the erosion mechanisms clear, it is important to control and to evaluate the particle impact conditions in a testing facility. A sand blast type erosion test rig, which can achieve the particle impact velocities up to 135 m s⁻¹ and a wide range of impact angles has been constructed. The key factors in particle impact conditions of particle flux, impact velocity and impact angle were examined. The relative distance between particles and particle size was discussed, as the particle flux affected erosion rate of material. A new method was proposed to determine particle velocities in this facility. The theoretical velocity of the particle calculated by the equations of particle motion was compared with the experimental results. The divergence of particles from a geometrical angle was evaluated by measuring surface roughness of the specimens. Although some spread of the particles were observed surrounding the central damage area of the specimen surface, the greatest amount of damage was concentrated in the center. As a result, it was found that particle impact conditions were well controlled in this testing unit.     

Erosion of elevated-temperature corrosion scales on metals

Combined erosion-corrosion poses a considerable problem to the design of long-lifetime metallic components in energy conversion systems. To gain some insight into this problem, scales were formed on stainless steel at elevated temperature and were subsequently eroded at room temperature to determine the nature of the erosion rates and the mechanism of scale removal. Thin corrosion scales were formed on stainless steel 310 and on an experimental Fe-18Cr-5Al-1Hf alloy at high temperatures (900 and 980°C) in gas mixtures with various levels of oxygen and combined O₂-S₂. The corroded specimens were eroded at room temperature in an air-solid particle stream using SiC particles 50 μm in size at a velocity of 60 m s^?1. The conditions of the corrosive exposures, the rates of erosion of these scales and the microscopic appearance of the eroded surface were correlated to determine the mechanism of thin scale erosion.     

Influence of particle impact conditions and temperature on erosion-oxidation of steels at elevated temperatures

The behaviour of four steels that are used in fluidized-bed boilers (16Mo3, T91, 304L and 253MA) has been studied in air (oxidation) and under impacts by sand particles in air (erosion-oxidation) at 350-650 °C in an extensive test programme and their performance compared in terms of the resulting weight changes and surface appearances.The results show that the oxidation rates for the steels increase with temperature but decrease with increases in chromium contents of the steels, as expected. Through oxidation rate, temperature has a significant influence on overall weight changes during combined erosion-oxidation, with material losses often increasing with temperature. The lowest particle velocities often cause deposition of particle debris and some weight gains, whereas the highest velocities cause essentially loss of material. Ductile erosion behaviour is observed under most conditions, with higher material losses at a shallow than at a steep impact angle, but the overall angle-dependency slightly changes with increase in temperature. The explanations for these observations are discussed in this paper. Furthermore, materials selection maps are constructed from the results generated, demonstrating that choosing the most highly alloyed material available is not necessarily the best rationale for materials selection.     

Mechanical characterization at intermediate strain rates for rate effects on an epoxy syntactic foam

An intermediate strain-rate mechanical testing technique was developed through proper modifications of a hydraulically driven loading frame (MTS 810) and a split Hopkinson pressure bar (SHPB). The modified MTS and SHPB were used to obtain valid stress–strain data for an epoxy syntactic foam at intermediate strain rates in the order from 10⁻¹ to 10² s⁻¹. Additionally, lower and higher strain-rate characterization of the foam material was conducted, such that the compressive stress–strain data of the syntactic epoxy foam were obtained at strain rates from 0.005 to 2150 s⁻¹ without any gap in the intermediate strain-rate range. The syntactic epoxy foam exhibited nonlinear strain-rate dependency of failure strength.