Cavitation resistance of Cr–N coatings deposited on austenitic stainless steel at various temperatures

Cr–N coatings were deposited on austenitic stainless steel, X6CrNiTi18-10, by means of the cathodic arc evaporation method at three substrate temperatures: 200 °C, 350 °C and 500 °C. All coatings were found to have a composition of Cr(N), CrN and Cr₂N. The substrate temperature was found to have an influence on the hardness and Young's modulus of the Cr–N coatings. The investigation of nanocrystalline Cr–N coatings resistance to cavitation was performed in a cavitation tunnel with a slot cavitator and tap water as the medium. The estimated cavitation resistance parameters of the coatings were the incubation period of damage and total mass loss. It was found that the optimal coating cavitation resistance was deposited at 500 °C. The incubation period for the 500 °C deposition coating was the same as that of the uncoated X6CrNiTi18-10 steel, but the total mass loss was significantly lower than on the uncoated specimen. The scanning electron microscope analysis indicated that the damage process of the Cr–N coating mainly originates from the plastic deformation of the steel substrate–hard coating system, which appears by “micro-folding” of the surface. An increase of tensile stresses at the top of micro-folds initiates micro-cracks and delamination of Cr–N coating. The results of the investigation and the analysis indicate that the factors mainly responsible for cavitation resistance of the steel substrate/hard coating system are resistant to plastic deformation of the total system and coating adhesion.     

Micro-scale abrasive wear testing of duplex and non-duplex (single-layered) PVD (Ti,Al)N, TiN and Cr–N coatings

A micro-scale abrasive wear test, based on ball-cratering, has been used to evaluate the wear resistance of duplex and non-duplex (Ti,Al)N, TiN and Cr–N coatings. The term duplex is used here when plasma nitriding is followed by PVD coating. Coatings without the plasma nitriding stage are termed single-layered. Coating properties were evaluated by surface profilometry, hardness and scratch testing. All duplex coatings showed higher micro-abrasive wear resistance than their single-layered counterparts, with the duplex (Ti,Al)N coating achieving the best performance. After a certain number of ball revolutions, the coating material became worn through, exposing the substrate material. After this point, the presence of a hard nitrided case diminished the scratching action of the SiC abrasive particles. The experimental results also indicate that the choice of the PVD coating plays an important role in improving the micro-abrasive wear resistance. Apart from single-layered and duplex Cr–N coatings, all the other coating systems provided a higher micro-abrasive wear resistance than the uncoated substrate (hardened AISI H13 steel). The poor abrasive wear resistance recorded for the single-layered and duplex Cr–N coatings could be attributed to the hardness of the Cr–N being much lower than that of the SiC abrasive particles, which caused tearing of the coating with subsequent delamination. The wear pattern observed was found to change from surfaces characterised by grooves (uncoated substrate, single-layered TiN and Cr–N systems and duplex Cr–N system) to surfaces which exhibited multiply indented surfaces (single-layered and duplex (Ti,Al)N systems), indicating a transition between wear mechanisms. This transition was found to be dependent on the ratio between the hardness of the SiC abrasive particles and surface (coating) or subsurface hardness. By decreasing this ratio, the ability of the SiC abrasive particles to scratch the composite surface was reduced and the resistance to micro-scale abrasion was improved.     

Cavitation erosion of NiAl-bronze layers generated by friction surfacing

Friction surfacing is a solid-state process, which allows deposition welding at temperatures below the melting range. For this investigation coating layers of NiAl-bronze were deposited by friction surfacing on self-mating substrates, followed by microstructural characterisation. Further, cavitation tests were performed in order to investigate wear resistance. Cavitation erosion mechanisms were analysed by means of optical and electron microscopy. All coatings show incubation periods about twice as long as those of the substrate material, while their average rate of material loss is about one half of that of the substrate. The differences in cavitation erosion resistance are due to more ductile behaviour of the coatings, as well as corrosion increasing the wear of the as-cast material.     

Cavitation damage incubation with typical fluids applied to a scroll expander system

During the operation of a scroll expander system overpressure may occur resulting in cavitation damage. Impacts due to implosion of cavitation bubbles near to suction ports can result in damage to the scroll plate in the expander. The accumulation of cavitation pits across the scroll plate leads to cavitation erosion hence efficiency drop. An experimental analysis to identify the mechanical damage of the cavitation on various steel surfaces with different liquid environments was conducted.Three liquid environments and four steel grades were utilised experimentally. The liquids used for the tests were distilled water, used as a reference liquid, and the two working fluids of the scroll expander a synthetic lubricant and a high molecular refrigerant. The steel grades were a high carbon (AISI 1085) and low carbon (AISI 1010) martensitic steel with retained austenite, a chromium martensitic steel (AISI 52100) and a martensitic scroll plate (SP) sample. An ultrasonic transducer was utilised to produce cavitation conditions using a 5 mm diameter probe. The comparison of the results revealed the most hostile liquid environment according to the morphology evaluation of the incubation pits. The cavitation mechanisms are discussed and the cavitation resistance of the steel grades is evaluated. The best performing steel material against cavitation is determined for the conditions described.     

Effect of liquid metal composition and hydrodynamic parameters on cavitation erosion

Cavitation erosion testing machine for low-temperature melting alloy liquid was developed by using a vibratory apparatus. The erosion tests of SUS304 were carried out in three kinds of lead–bismuth and deionized water. We defined a relative temperature as the percentage between freezing and boiling points. At relative temperature at 14 °C, the erosion rate is 10–12 times in various lead–bismuth alloys, and 2–5 times in sodium, as compared with that in deionized water. When SUS304 was exposed to a cavitation in PbBi, the surface was work hardened 20% harder compared with original surface. In deionized water, SUS304 was work hardened by 5%. Therefore, we can conclude that larger collapse pressure can be estimated to act on the specimen surface in lead–bismuth, as compared with that in water.We discussed the effect of hydrodynamic properties on cavitation erosion in a flowing system. It is considered that the erosion rate in sodium is in proportion to 1st to 6th power of flow velocity similarly to that in mercury. The incipient cavitation number is approximately unity irrespective of test liquids. Furthermore, the relation between MDER and cavitation number is expressed as power low of function with an exponent of 2.5.     

Cavitation erosion characteristics of nickel-based alloy-composite coatings obtained by plasma spraying

Nickel-based alloy and composite coatings were obtained on an 18-8 stainless steel substrate by a plasma spraying technique. They were sintered in a vacuum furnace (10^?2 Pa) to improve toughness and to reduce hardness. The cavitation erosion resistance of these coatings was evaluated using a rotating disc apparatus. The results of the investigation reveal that sintered coatings are several times better than “as-sprayed” plasma coatings. The sintered coatings can help in increasing the length of the incubation period.     

Erosive wear of hardfaced Fe–Cr–C alloys at elevated temperature

Many engineering components are subjected to erosive wear at elevated temperature. As erosive wear at elevated temperature is governed by the synergistic effect of erosive wear and oxidation, it is possible to modify surfaces of the components in order to achieve improved performances. In view of the above, two different types of hardfacing alloys of Fe–Cr–C were designed incorporating Nb, Mo and B to ensure improved performances at elevated temperature. In order to achieve the above objective, mild steel was hardfaced with these alloys under optimised gas metal arc welding (GMAW) condition. The microstructures of the hardfaced coating was characterised with the help of optical microscopy (OM) and scanning electron microscopy (SEM). The mechanical properties of these coatings were obtained by means of micro indenter. Erosive wear of these coatings was evaluated for four different temperatures, for two different impact angles and at one impact velocity. The morphologies and the transverse sections of the worn surfaces are examined with SEM. The erosive wear of these coatings were compared with conventional M2 tool steel. Results indicate that erosion rate of these coatings increases with increase of test temperature and impact angles. Among various coatings, Fe–Cr–C coating containing higher amount of Nb, Mo and B exhibits best erosion resistance particularly at elevated temperature.     

Effect of heat treatment on the structure,cavitation erosion and erosion–corrosion behavior of Fe-based amorphous coatings

In this study, high-velocity oxygen-fuel sprayed amorphous coatings have been heat treated at various temperatures to form microstructures with crystalline phases. The structure, micro-hardness, cavitation erosion resistance and erosion–corrosion resistance of these coatings are compared. Crystalline phases are discovered in the coatings after heat treatments at 650 °C and 750 °C. The coating heat treated at 750 °C exhibits the poorest cavitation erosion resistance in 3.5 wt% NaCl solution among all coatings due to the degraded corrosion resistance. However, the hardness of the crystallized coating can reach 1000 Hv and the erosion–corrosion resistance of the heat treated coating is better than the untreated one.     

The cavitation behavior of a metastable Cr–Mn–Ni Steel

The cavitation behavior of metastable Cr–Mn–Ni steel was investigated in a rotating disk machine. The microstructure of the cavitated area was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical microscopy. The results of XRD showed that the γ→ phase transformation and ηNi₃Ti precipitation had occurred during cavitation testing. Cavitation damage of the material occurred by combined thermal–mechanical fatigue failure. It was deduced that the high-temperature behavior and properties should be taken into account in evaluating the cavitation resistance of such materials.     

Relations between cavitation erosion resistance of materials and their fatigue strength under random loading

The paper contains results of tests on fatigue strength under uniaxial random loading and cavitation erosion resistance for three steels: 10HNAP, 18G2A and 15G2ANb. The obtained fatigue and cavitation characteristics were used for determination of relations between these two phenomena. From the analysis it appears that there is correlation between fatigue strength of the material under random loading and its cavitation erosion resistance. It has been shown that fatigue tests under random loading and tests on cavitation erosion of 10HNAP, 18G2A and 15G2ANb steels may be described with a mathematical model of the same type. It has been also found that there is a linear relation, in the dual logarithmic system, between cavitation erosion resistance of the steels tested and their fatigue strength under random tension—compression with zero mean value.     

The Influence of W-DLC and CrxN Thin Film Coatings on Impact Damage between Bearing Materials

In this work, the effect of hard tribological coatings was studied in terms of mitigating impact damage between tungsten carbide spherical elements and two different types of steel substrates. The coatings included a hard, highly elastic Tungsten-incorporated diamond-like carbon (W-DLC) coating at two different thicknesses and a harder, less elastic Cr_xN coating. Impacts were created using a drop-test rig described herein and characterized in three ways: a measure of the coefficient of restitution during impact, investigation of the impact site using an optical interferometer, and fixed ion beam cross sections of select impacts for observation of subsurface damage within the coating and substrate. It was found that hard coatings on softer substrates such as 440C steel were able to mitigate surface damage up to a certain impact speed, depending on the coating, but were unable to influence the coefficient of restitution. On harder substrates like 52100 alloy steel, the coatings were found to increase the coefficient of restitution, indicating a reduction in energy loss due to plastic deformation, and to reduce damage at each tested speed. These effects and their potential influence on bearing performance are discussed in regard to impact mechanics, surface metrology, and the material properties of the coating and substrate acquired by nanoindentation.     

Cavitation resistance, microstructure and surface topography of materials used for hydraulic components

The cavitation resistance of a stainless steel and two flame thermal spray coatings was tested in laboratory according to ASTM G32 standard. The time-variation curves of cumulative volume loss, erosion rate and roughness parameters were related to the microstructure of the samples and to the wear mechanisms. Pores, unmelted particles and other microstructure defects prevented the coatings from showing an incubation period during the tests, while the stainless steel exhibited the expected incubation, acceleration and maximum rate stages. In the stainless steel, a correlation between the transition from incubation to acceleration stage and the R_sm/R_q and R_y/R_q ratios was established.     

Cavitation damage incubation with typical fluids applied to a scroll expander system

During the operation of a scroll expander system overpressure may occur resulting in cavitation damage. Impacts due to implosion of cavitation bubbles near to suction ports can result in damage to the scroll plate in the expander. The accumulation of cavitation pits across the scroll plate leads to cavitation erosion hence efficiency drop. An experimental analysis to identify the mechanical damage of the cavitation on various steel surfaces with different liquid environments was conducted.Three liquid environments and four steel grades were utilised experimentally. The liquids used for the tests were distilled water, used as a reference liquid, and the two working fluids of the scroll expander a synthetic lubricant and a high molecular refrigerant. The steel grades were a high carbon (AISI 1085) and low carbon (AISI 1010) martensitic steel with retained austenite, a chromium martensitic steel (AISI 52100) and a martensitic scroll plate (SP) sample. An ultrasonic transducer was utilised to produce cavitation conditions using a 5 mm diameter probe. The comparison of the results revealed the most hostile liquid environment according to the morphology evaluation of the incubation pits. The cavitation mechanisms are discussed and the cavitation resistance of the steel grades is evaluated. The best performing steel material against cavitation is determined for the conditions described.     

湿式高速车铣时TiN涂层刀具的磨损机理研究

研究了在水溶性冷却液浇注冷却条件下，用TiN涂层刀具高速车铣D60钢时刀具的磨损机理和磨损特性。切削过程中涂层剥落是造成刀具损坏的主要原因。涂层剥落后，切削区产生的瞬时热量聚集使基体粘结相软化，从而导致硬质相颗粒脱落和基体的剧烈磨损。     

Estimation of Incubation Time of Cavitation Erosion for Various Cavitating Conditions

Estimations have been made, resulting in a general method for the prediction of the incubation time for cavitation erosion using various cavitating conditions and materials. From a single erosion test, the incubation time can be estimated for various conditions and materials by plotting the mass loss as a function of exposure time to cavitation on a log–log scale.     

耐泥沙磨蚀涂层研究

分析了热喷涂层试验室模拟及水轮机真机运行的试验结果。选择出耐磨性好的合金粉末和喷涂工艺，以提高水轮机和水泵的抗磨蚀性能，研究成果成功应用于引黄大泵生产，避免了腐蚀破坏。     

Fretting and fretting corrosion behavior of novel micro alloyed rail steels

The fretting behavior of two novel rail steels of composition Cu–Ni and Cr–Cu–Ni has been studied and compared with that of C–Mn and Cu–Mo rail steels. The rail steels were fretted in dry condition and in 3.5% NaCl solution. All the four rail steels exhibited similar fretting behavior in dry condition. The morphological features of the wear scar surface indicated delamination process as the main wear mechanism. The wear damage in 3.5% NaCl was lower compared to that in dry condition for all the rail steels due to lubricating effect of the solution. Fretting in presence of 3.5% NaCl resulted in lower wear volume for Cu–Ni and Cr–Cu–Ni rail steels. The friction coefficient for Cr–Cu–Ni rail steel was lower than that of C–Mn, Cu–Mo and Cu–Ni rail steels. Scanning electron microscopy (SEM) showed better adherence of tribo-electro-chemical layers formed on Cu–Ni and Cr–Cu–Ni rail steel than the C–Mn and Cu–Mo rail steels. This has been related to the improved fretting corrosion behavior of Cu–Ni and Cr–Cu–Ni rail steels.     

Influence of temperature on erosion by a cavitating liquid jet

The influence of water temperature on cavitation erosion has previously been studied using a vibratory apparatus, but no researches have been conducted at a constant cavitation number in flow condition. This study deals with the influence of the water temperature on cavitation erosion using a cavitating jet apparatus. The optimum stand-off distance at 25 °C was 11, 15, 21 and 25 mm at cavitation number of σ = 0.03, 0.025, 0.02 and 0.015, respectively, and was almost the same as that of a guideline in the ASTM G134 standard. The optimum stand-off distance at 75 °C is similar to that at 25 °C. The erosion rate increases with the liquid temperature and reaches a peak, followed by a decrease. The relative temperature was defined as 0 °C for freezing temperature and 100 °C for boiling temperature of pressurized water. The peak appears at the approximate average of freezing and boiling relative temperatures. The erosion rate increases by 1%/ °C between 5 and 45 °C of relative temperatures, and decreases by 2%/ °C between 45 and 80 °C.     

Solid Particle Erosion Behavior of WC Coating Obtained by Electrospark Technique and Detonation Spraying

Solid particle erosion is an important material degradation process. One way of improving the erosion resistance of a material is to suitably modify the surface. Electrospark deposition (ESD) is a well-known surface modification process. Operational simplicity, low capital cost, and low operational cost of the ESD process have made it attractive for high-technology areas in engineering industries. Tungsten carbide (WC) is considered a potential hard material for erosion-resistant application. This material can be deposited by ESD. The present investigation has been undertaken to evaluate the room-temperature erosion response of WC coating deposited by ESD and to compare the erosion behavior of this coating with that of detonation-sprayed WC-Co coating. WC coatings were deposited on mild steel (MS) and aluminum substrate by ESD. Similarly, WC-12% Co coatings were deposited on MS and Al by detonation spraying. The microstructural features and mechanical properties of these coatings were characterized using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, and microhardness testing. The solid particle erosion rate was determined using an erosion test rig. The morphology of the eroded surfaces and the areas beneath the eroded surfaces were examined by means of SEM. The results showed that the WC coating by ESD improves erosion resistance. Although most coatings exhibit a ductile erosion response, WC coating by ESD on Al substrate exhibits a brittle erosion response. Material loss from ESD coating on Al occurs due to the joining of preexisting cracks and the removal of chunk of material.     

High-temperature low cycle fatigue, creep–fatigue and thermomechanical fatigue of steels and their welds

High-temperature low cycle fatigue (LCF) is influenced by various time-dependent processes such as creep, oxidation, phase transformations and dynamic strain ageing (DSA) depending on test conditions of strain rate and temperature. In this paper, the detrimental effects of DSA and oxidation in high-temperature LCF are discussed with reference to extensive studies on 316L(N) stainless steel and modified 9Cr–1Mo steel. DSA has been found to enhance the stress response and reduce ductility. It localizes fatigue deformation, enhances fatigue cracking and reduces fatigue life. High-temperature oxidation accelerates transgranular and intergranular fatigue cracking in modified 9Cr–1Mo steel and during long hold time tests in austenitic stainless steel. In welds, microstructural features such as presence of course grains in the HAZ and formation of brittle phases due to transformation of δ ferrite during testing influence crack initiation and propagation and fatigue life. Thermomechanical fatigue (TMF) studies are suggested as more closer to the actual service conditions. In 316L(N) stainless steel, TMF lives under out-of-phase cycling are found to be lower than those under in-phase conditions in the low-temperature regimes, while the converse holds good when the upper temperature encompassed the creep-dominant regime.