The effect of nitriding, severe shot peening and their combination on the fatigue behavior and micro-structure of a low-alloy steel

The wear and corrosion resistance of shot peened–nitrided 316L austenitic stainless steel

Shot peening treatment is a well-known and practical method to improve surface properties of materials. In this method, surfaces of materials are peened by small steel shots and thus, surface hardening is provided. Although it is used in a wide range of applications, this process could affect the electrochemical behavior of materials because of changing surface properties. Therefore, the effects of shot peening process on the electrochemical properties of a low-alloy steel were examined in the present study. AISI 4140 low-alloy steel samples were shot peened in different intensities of 16 A, 18 A, 20 A and 24 A. Afterwards, corrosion tests were carried out at room temperature in a 3.5% NaCl solution. Open circuit potential (OCP) electrochemical polarization and electrochemical impedance spectroscopy (EIS) analysis were performed in corrosion tests. The microstructural, morphological and surface properties of samples were analyzed by XRD, SEM and 3D surface profilometer. The structural analyses showed that grain structure of the material was affected by shot peening treatment. A plastically deformed zone, which have extended and refined grain structure, formed after shot peening processes. Electrochemical analyses indicated that the corrosion resistance of the material increased with the increasing shot peening intensity owing to grain refinement and formation of sub-grains. Also, examinations on the corroded surfaces showed that crevice corrosion was the main mechanism for shot peened samples.

The effect of surface plastic deformation produced by shot peening on corrosion behavior of a low-alloy steel

The present study addresses the effect of multiple jet passes and other parameters namely feedrate and pressure in waterjet peening (WJP) of austenitic stainless steel 304. An analysis of surface integrity was used to evaluate the performance of different parameters in the WJP process. An increase in the number of jet passes as well as pressure leads to a higher roughness and more erosion of the surface. However, the feedrate shows a reverse effect on the surface roughness and erosion. The surface microstructures also show the mechanism of material removal process involving initial and evolved damages. The subsurface hardness shows that treating the surface with a higher number of passes and pressure produces a higher increase of hardness and also a deeper hardening layer. But, a reverse effect on the subsurface hardness was found for the feedrate. Furthermore, cross-sectional microstructures show a higher density of slip bands in the deformed grains of the specimen treated with a higher number of jet passes and pressure. However, the amount of slip bands in the deformed grains is lower with increasing feedrate.

Improving surface hardness of austenitic stainless steel using waterjet peening process

A severe plastic deformation (SPD) surface layer was introduced by shot peening to enhance the nitriding kinetics in low-temperature (600 °C) plasma nitriding of Ti–6Al–4V alloy. The effect of this pretreatment on the nitrided microstructures and phase compositions was investigated by analytical microscopy techniques e.g. scanning and transmission electron microscopy (SEM, TEM) and X-ray diffraction (XRD) analysis. Microstructural investigations revealed the formation of a compound layer consisting of a 0.6 µm thick nanocrystalline TiN layer followed by a 0.5 µm thick layer of Ti2N with a larger grain size (0.1–0.5 µm). The development of TiN nanograins was attributed to accelerated nitriding kinetics due to the increased preferential nucleation sites in the SPD layer. Furthermore, the thickness of nitrogen diffusion zone (DZ) increased by 50% in the pretreated plasma nitrided alloy when compared with that of the untreated one. This is likely promoted by an increase in density of subsurface microstructural defects, such as twins and grain boundaries. The sliding behaviour and interfacial adhesion of the nitrided surfaces were evaluated by micro-scratch tests within a load range of 1–20 N. Compared with untreated-plasma-nitrided alloy, the pretreated nitrided surfaces exhibited a higher load bearing capacity and better interfacial bonding. They exhibited no chipping or spallation, even after multiple sliding passes at the highest applied load of 20 N in contrary to the untreated plasma nitrided surfaces.

Effect of SPD surface layer on plasma nitriding of Ti–6Al–4V alloy

AISI 304 austenitic stainless steel was plasma nitrided at the temperature ranging from 410 to 520 °C with pre-shot peening. The structural phases, micro-hardness and electrochemical behavior of the nitrided layer were investigated by optical microscopy, X-ray diffraction, micro-hardness testing and anodic polarization testing. The effects of shot peening on the nitride formation, nitride layer growth and corrosion properties were discussed. The results showed that shot peening enhanced the nitrogen diffusion rate and led to a twice thicker nitrided layer than the un-shot peening samples under the same plasma nitriding conditions (410 °C, 4 h). The nitrided layer was composed of single nitrogen expanded austenite (S-phase) when nitriding below 480 °C, which had combined improvement in hardness and corrosion resistance.

Plasma nitriding of AISI 304 austenitic stainless steel with pre-shot peening

The nitriding of low alloy steel has been carried out at anodic potential in a space enclosed by an active screen that consists of two cylinders with different diameter. These two cylinders made up a hollow cathode in a discharge system. The difference in diameter of the two cylinders is about 8–10 mm to maintain strong discharge between them. They can also be heated rapidly to the required temperature for treatment. The sample to be nitrided was held at the same potential as that of the anode used in the discharge and heated through heat radiation from the hot cylinders and by electron bombardment. Electrons bombarded the surface of the sample even though the intensity of bombardment was low because of the anodic sheath. To illustrate the effect of the anodic potential on the nitriding a comparison was made between nitriding at anodic and cathodic potential (general plasma nitriding). The phase composition, the compound layer thickness and the surface topography of the nitrided layer, as well as its properties, were investigated using X-ray diffraction, scanning electron microscopy and microhardness tester. In particular, the corrosion properties of the untreated and plasma nitrided samples were evaluated using anodic polarization tests in 3.5% NaCl solution. The results showed that anodic plasma nitriding not only increased the surface hardness but also improved the corrosion resistance of the low alloy steel.

Plasma nitriding of 42CrMo low alloy steels at anodic or cathodic potentials

The invention discloses a composite processing device and a processing method for nitriding and oxidizing the surface of low-alloy steel. The device comprises a vacuum furnace, an air inlet system, a vacuum pumping system, a power supply system and a temperature measurement system, wherein a hollow cathode auxiliary device and a workpiece platform are installed in the vacuum furnace, the hollow cathode auxiliary device comprises a small cylinder and a big cylinder which have different diameters and are provided with holes, the small cylinder and the big cylinder are respectively connected with the cathode of a power supply, and the small cylinder is provided with an insulation ceramic wafer; and the workpiece platform comprises a supporting seat and an object stage, wherein the supporting seat is positioned on a base of the vacuum furnace, and the object stage is positioned on the supporting seat, used for placing a workpiece and connected with the anode of the power supply. By the placement of the hollow cathode device, the invention enhances the discharge strength and the ionization rate by hollow cathode discharge, thereby generating a penetration and diffusion substance with high concentration and high activity and forming a modification layer with good corrosion resistance, high hardness and favorable properties of friction and abrasion at the surface.

Composite processing device and processing method for nitriding and oxidizing surface of low-alloy steel

The invention discloses a surface oxidation treatment device and method of aluminum alloy. The device comprises a vacuum furnace, an air inlet system, a vacuum-pumping system, a power supply system, a temperature detecting system, and a cooling system; the vacuum furnace is internally provided with a workpiece platform and a hollow cathode auxiliary device; the workpiece platform comprises a supporting seat, an objective table, an insulator, wherein the objective table is connected with a anode power supply; the hollow cathode auxiliary device comprises a big cylinder and a small cylinder which are coaxial and different in diameters and have holes; and the big cylinder and the small cylinder are respectively connected with a cathode power supply. The method mainly comprises the step of adjusting the space between the big cylinder and the small cylinder to enable the double-layer cylinder to keep the hollow cathode discharging. When glow discharging occurs inside the vacuum furnace, electrons vibrate between cathodes of the two cylinders and alternate to acquire energy from sheath layers of the two cathodes to generate arc discharging, i.e. high-concentration high-activity permeation materials generated from cathode discharging rapidly saturate and reactively diffuse on the workpiece surface through holes distributed on the small cylinder to enable the workpiece to acquire a high-quality strengthened layer.

Surface oxidation treatment device and method of aluminum alloy

The utility model discloses a low-alloy steel surface nitriding-oxidizing composite processing device, which comprises a vacuum furnace, an air intake system, a vacuum-pumping system, a power supply system and a temperature detecting system, wherein a hollow cathode auxiliary device and a workpiece platform are arranged in the vacuum furnace; the hollow cathode auxiliary device consists of a small cylinder and a big cylinder; the cylinders have different diameters and are provided with holes; the small cylinder and the big cylinder are respectively connected with the cathode of a power supply; the small cylinder is provided with an insulating ceramic plate; the workpiece platform comprises a support and an objective table; the support is positioned on the base of the vacuum furnace; the objective table is positioned on the support; and the workpiece platform is used for placing workpieces and connecting the cathode of the power supply. In the utility model, due to the arrangement of the hollow cathode device, the hollow cathode is used for discharging, thereby improving the discharging strength and the ionization rate, so as to generate penetration and diffusion matter with high concentration and high activity; moreover, a modified layer with good corrosion resistance, high hardness and good friction-abrasion property.

Chunhua Wang

Papers

Plasma nitriding of AISI 304 austenitic stainless steel with pre-shot peening

Plasma nitriding of 42CrMo low alloy steels at anodic or cathodic potentials

Composite processing device and processing method for nitriding and oxidizing surface of low-alloy steel

Surface oxidation treatment device and method of aluminum alloy

Low-alloy steel surface nitriding-oxidizing composite processing device