Microstructural feature of laser surface melting 1Cr17Ni2 stainless steel

M42C stainless steel coatings were deposited on the surface of cast iron through atmospheric plasma spray with different plasma arc powers of 30 kW, 35 kW and 40 kW to obtain higher oxidation resistance than that of cast iron. The high temperature oxidation tests were carried out at 400 °C, 530 °C and 700 °C. It was found that the oxidation mass gains of the M42C coatings were much lower than those of cast irons during initial oxidation. With the increase of oxidation temperature and time, the oxidation mass gains of cast irons increased much more quickly than those of the M42C coatings. At 400 °C and 530 °C, the oxidation mass gains of the M42C coatings were higher than that of the M42C bulk. When the temperature reached 700 °C, the oxidation mass gains of the M42C coatings were just a little higher than that of the M42C bulk. Chromite spinel FeCr₂O₄ were formed which could hinder the further oxidation in the M42C coating. During the service of the M42C coatings at 700 °C, the oxidation resistance of the M42C coating was increased to the level close to that of the bulk.

The application of laser surface treatment to improve the corrosion resistance of ferrous alloys is discussed. By dissolving precipitates of phases present in small proportions in the alloy’s microstructure (carbides, nitrides, intermetallic compounds, etc.) or preventing their precipitation during cooling, laser surface treatment improves, in general, the localised corrosion resistance of ferrous alloys, in particular of stainless steels. However, in order to achieve this improvement, chemical and microstructural inhomogeneities resulting from the laser treatment process must be avoided.

Biomimetic surface is an effective ways to promote the performance grade and applied range of materials without altering their substrate. Many improved properties such as resisting fatigue, enduring wear, etc, have been achieved by applying biomimetic morphology or structure to some engineering material surfaces. In this paper, aiming to reveal the relationship between thermal cracking behavior and mechanical properties of engineering materials with biomimetic surface, biomimetic specimens were fabricated using laser technique by imitating the heterogeneous structure on the surface of plant leaves. The effect of thermal fatigue cycling on the tensile properties of H13 die steel specimens with different surfaces (several types of biomimetic surfaces and a smooth surface) was compared and investigated. As a result, due to the coupling effects of the morphological features on the surface and the microstructure characteristics within unit zone, these specimens with biomimetic surface exhibit remarkably enhanced Ultimate Tensile Strength (UTS) and 0.2% Yield Strength (YS) compared with reference specimens while corresponding ductility remains largely unaffected even heightened, whether the thermal fatigue loads or not. The relative mechanisms leading to these improvements have been discussed.

Biomimetic coupling surface is a recent development in surface science of materials. One of the important methods to achieve this surface is to manufacture large numbers of structural units by certain techniques, arranging them regularly on the surface layer of materials to form the biomimetic structure. The penetration depth and the surface roughness of units are two crucial factors that affect strongly the properties of materials. In this paper, a YAG pulsed laser with varied parameters (electrical current 200 A to 300 A, pulse duration 5 ms to 15 ms, frequency 4 Hz to 10 Hz and scanning speed 0.24 mm·s⁻¹ to 0.72 mm·s⁻¹) was used to fabricate these units on the surface of 3Cr2W8V die steel. The penetration depth and surface roughness of the units were investigated based on orthogonal experimental design. To maximize the penetration depth and minimize the surface roughness, the range analysis and subsequently overall balance method were adopted to identify the most significant factor and level. Meanwhile the most preferable combination of the laser processing parameters was selected. The effect of laser processing parameters on the penetration depth and surface morphology of units was analyzed. The interrelationship among the processing parameters, the penetration depth and the surface roughness was discussed.

The stabilisation of retained austenite is investigated in a laser surface melted martensitic stainless tool steel. By comparing experimental results with the results obtained from a simple semi-empirical model, it is concluded that the microstructural refinement caused by laser melting is, most probably, the cause for the stabilisation at room temperature of large proportions of retained austenite in laser surface melted tool steels. As a consequence, the proportion of retained austenite in laser surface melted tool steels can be minimised by avoiding the use of high laser scanning speeds, which results in the formation of extremely fine dendritic structures.

The surface modification of an H13 tool steel using a tungsten arc heat source was studied. The investigation shows that, when surface melting was performed in a controlled atmosphere of N₂, CO₂ or CO₂–Ar mixture, the wear resistance of the surfaces was significantly enhanced over the untreated steel. The most wear-resistant surface was that for N₂ and the least for surfaces melted in a CO₂–Ar mixture. The resolidified surface microstructure consisted primarily of δ ferrite with some retained austenite and the increase in wear resistance was attributed to an increase in surface hardness due to the formation of carbo-nitride precipitates within the modified surfaces.