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The present work deals with fabricating a carbide reinforced nickel matrix composite coating on 4Cr5MoSiV1 steel. Electroless nickel coated silicon carbide particles (Ni/SiCp) were injected into plasma generated nickel alloy melt pool using plasma melt injection (PMI) method and plasma transfer arc (PTA) process simultaneously. The appropriate injection angle (θ) in this comprehensive PMI method (PTA + PMI) was calculated to satisfy the specific requirement of injection position for reducing the gasification of SiC particles (SiCp). The effects of the PTA + PMI method and electroless nickel coating (ENC) on SiCp were identified in a comprehensive manner by comparing the microstructural aspects of the coatings. The results show that the appropriate value of θ ranges from 57° to 64° in the present study. The deposition of ENC on SiCp is found to be very effective in retarding the gasification of SiCp during the PTA + PMI process. Meanwhile, the improved wettability of SiCp by the ENC decreased the necessary minimal vertical velocity of SiCp, which consequently promoted the impregnation of SiCp in the PTA + PMI process. The nickel matrix composite coating reinforced by Ni/SiCp (S5) using the PTA + PMI method mainly consists of large amount of in situ chromium-rich carbides (M7C3) together with nickel silicide (Ni3Si) in a nickel iron solid solution (γ-Ni(Fe)). Microhardness and tribological tests reveal considerable improvement in surface microhardness and wear resistance properties of 4Cr5MoSiV1 steel.

Niobium carbide coating was produced by thermal-reactive diffusion technique on AISI 52100 steel in salt bath at 1 123 K, 1 173 K, and 1 223 K for 1, 2, 4, and 6 hours. The salt consisted of borax, sodium fluoride, boron carbide, and niobium pentoxide. The presence of NbC phase on the steel surface was confirmed by X-ray diffraction analysis. Microscopic observation showed that niobium carbide coating formed on the substrate was smooth and compact. There was a distinct and flat interface between the coating and substrate. The micro-hardness of niobium carbide coating was 2892±145HV. The thickness of coating ranged from 1.6 μm to 14μm. The forming kinetics of niobium carbide coating was revealed. Moreover, a contour diagram derived from experimental data was graphed for correct selection of process parameters. Some mathematical equations were built for predicting the coating thickness with predetermined processing temperature and time. The results showed that these mathematical equations are very practical as well as the kinetics equation.

To meet the performance requirements of hot forging die heat resistant layer, the Ni60-SiC coating, Ni60-Cr3C2 coating, and Ni60-WC coating were prepared using W6Mo5Cr4V2 as substrate material with 30%SiC, 10%Cr3C2, 30%WC powder by means of plasma spraying and plasma spray re-melting and plasma spray welding, respectively. Microstructure of each carbide coating was analyzed, micro-hardness was tested, and mainly thermal parameters of coating were detected. The experimental results show that using plasma spray welding, the performance of 70%Ni60/30%SiC powder is the best, and its micro-hardness can achieved 1100HV, showing good thermal-physical property.

In order to produce the hear-resistant inner layer of hot-forging die, the plasma spraying and plasma re-melting and plasma spray welding were adopted. Substrate material was W6Mo5Cr4V2, including 10%, 20%, 30% tungsten carbide (WC) ceramic powder used as coating material to obtain different Nickel-based WC alloys coating. Micro-structure and micro-hardness analysis of the coating layer are conducted, as well as thermophysical properties for the coating layer were measured. The experimental results show that the coating prepared with 70%Ni60, 30%WC powder has the best properties with plasma spray welding, in which the micro-hardness can achieve 900HV, meanwhile it can improve the thermal property of hot-forging die dramatically.

Vanadium pentoxide, borax, boron carbide and sodium fl uoride were used to grow vanadium carbide coating on surface of Cr12 steel at 950 °C by TD process. The coating of vanadium carbide (VC) extended the serve-life period of Cr12 steel as punching die. Kinetics of vanadium carbide coating growth was brought forward and verified by comparison of the mathematical model with the experimental results. The thickness of coating was illustrated by SEM. The chemical constituent of coating and remnants were tested by X-ray diffraction (XRD) and X-ray energy dispersive spectroscopy (EDS). To increase the thickness, rare earth silicon powder (FeSiRe23) was added to the borax salt bath. The analysis of XRD revealed that FeSiRe23 increased the depth of vanadium car-bide coating as reducing agent and catalysis.

In order to prepare heat-resistant inner layer of hot-forging die, plasma spraying, plasma remelting and plasma spray welding were adopted. Cr3C2 coatings of Ni-Based were prepared respectively with 10%, 20% and 30% Cr3C2 powder and W6Mo5Cr4V2 substrate. The coating microstructure analysis, the micro-hardness test, and the measurement of thermal parameters of coating were conducted. The experimental results show that the coating has the better thermo-physical property by using plasma spray welding method with the powder ratio of 90% Ni60 and 10% Cr3C2, and by this way the micro-hardness of coating can achieve 1100 HV.

The segregation of thermal diffusion salt bath chromizing process was analyzed. The experimental chromizing ingredients were prepared by the four groups A, B, C, and D. In order to study the segregation status of this case, the cooling molten salt in the crucible was removed by drilling from the heart core of molten salt. The core of molten salt was analyzed by X-ray fluorescence spectroscopy and XRD. Through the analysis, we can conclude that the Cr element deposited in the bottom was 4.51 times than the top. Chloride added to the molten salt will reduce segregation. Meantime we proposed some measures to overcome the segregation problem.

In order to produce the hear-resistant inner layer of hot-forging die, plasma spraying and plasma re-melting and plasma spray welding were adopted. Substrate material was W6Mo5Cr4V2, including 10%, 20%, 30% SiC ceramic powder used as coating material to obtain different Ni-based SiC alloys coating. Micro-structure and micro-hardness analysis of the coating layer were followed, as well as thermophysical properties for the coating layer were measured. The experimental results show that the coating prepared with 70% Ni60, 30% SiC powder has best properties with plasma spray welding, in which the micro-hardness can achieve 1100 HV, meanwhile can improve the thermal property of hot-forging die dramatically.

To satisfy performance and long life requirements for hot forging die, Ni60-Cr3C2 composite coatings were prepared on the high-speed steel W6Mo5Cr4V2 using laser cladding technology. Laser clad coatings with different ratios of Ni60:Cr3C2 were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX) and microhardness tester, respectively. Specific heat capacity and thermal conductivity were measured by Laser Thermal Constant Meter. Thermal expansion coefficient and elastic modulus were measured by Dynamic Mechanical Thermal Analyzer and Electro-Hydraulic Servocontrolled Testing System, respectively. The results indicated that Ni60+50wt% Cr3C2 composite coating had dense and homogeneous structure, as well as a metallurgical bonding with the substrate. With the increase of Cr3C2 content, volume of chromium-containing compounds in the composite coating increased, microhardness increased and microstructure refined. The thermal physical parameters results showed that Ni60+ 50wt% Cr3C2 composite coating was overall worse than W6Mo5Cr4V2, but had a higher hot yield strength to alleviate hot fatigue and surface hot wear of hot forging die during hot forging and thus improve the service life of hot forging die.

Vanadium pentoxide, borax, boron carbide and sodium fluoride were used to grow vanadium carbide coating on die steel(Cr12, Cr12MoV) surface at 950 °C by TD process, which extended the life period of Cr12 and Cr12MoV as punching die. Kinetics of vanadium carbide coating growth was brought forward and verified by comparison of the mathematical model with the experimental results. The thickness of coating was illustrated by SEM. The chemical constituent of coating and remnants were tested by XRD. To increase the thickness, rare earths (FeSiRe23) were added to the borax salt bath. The electronic probe microanalysis (EPMA) revealed that the addition of rare earths could decrease carbon concentration in the coating and increase the depth of vanadium carbide coating.

A new design technique for the long life hot forging die has been proposed. By finite element analysis, the reason for the failure of hot forging die was analyzed and it was concluded that thermal stress is the main reason for the failure of hot forging die. Based on this conclusion, the whole hot forging die was divided into the substrate part and the heat-resistant part according to the thermal stress distribution. Moreover, the heat-resistant part was further subdivided into more zones and the material of each zone was reasonably selected to ensure that the hot forging die can work in an elastic state. When compared with the existing techniques, this design can greatly increase the service life because the use of multi-materials can alleviate the thermal stress in hot forging die.

The corrosion of stainless steel was experimentally investigated and analyzed to improve the service-life of the crucible and fixture clamps. Through the experiment, stainless steel was found to satisfy the crucible and fixture clamps materials. As the chromium element mass percentage of the stainless steel increased, the corrosion decreased rapidly at first and then slowly increased. With the corrosion time prolonging, the corrosion growth rate is generally a downward trend. With Chromium 20%, the corrosive were the least. With Nickel element mass percentage increased, the corrosion increased rapidly at first and then decreased rapidly, flatten at the last. It was special that the corrosion had little relationship with the Chromium and Nickel mass percentage when the Nickel percentage is more than 35%. The most remarkable corrosion is corresponding with the Nickel element 12%, the least corrosion with Nickel element 80%. So Nickel element 35% is the most optional.

The service life of hot forging die is mainly decided by Stresses, including thermal, mechanical and combined stress. Stress amplitude is affected by the material properties, such as specific heat capacity. The choice of die material is very importance for improving die service life. In order to identify the effect of specific heat capacity on die stress, a 2D FE model of cake shaped workpiece is developed to analyze thermal, mechanical and combined stress of hot forging die with different level specific heat capacity. Three kinds of stress distribution are explored. The results show that thermal stress is about 10 times than mechanical stress at the die cavity surface which is the easiest failure location, and approximate linear relationship exists between stress, including thermal and combined stress, and specific heat capacity. Die material with large room temperature and temperature-sensitive specific heat capacity should be selected.

In order to improve service life of hot forging die, Ni60-SiC cermet coating was prepared on the high speed steel W 6 Mo 5 Cr 4 V 2 under different processing parameters using plasma cladding technology. The clad coating composition, microstructure and microhardness were analyzed by X-ray diffractometer (XRD), optical microscopy (OPM) and microhardness tester. The experimental results show that Ni+25 wt. % SiC composite coating with pre-placed layer thickness T=3mm has the best metallurgical bonding and best microhardness profile.

Stresses, including thermal, mechanical and combined stress, are the root causes leading to hot forging die failure. Stress amplitude is affected by the material properties, such as elastic modulus. In order to select die material for improving die service life, a 2D FE model of cake shaped workpiece is developed to analyze thermal, mechanical and combined stress of hot forging die with different level elastic modulus. Three kinds of stress distribution are explored. The results show that thermal stress is about 10 times than mechanical stress at the die cavity surface which is the easiest failure location, and approximate linear relationship exists between stress, including thermal and combined stress, and elastic modulus. Die material with large room temperature and temperature-sensitive elastic modulus should be selected.

In order to satisfy performance and long life requirements for hot forging die, Ni60-SiC cermet coating was prepared on the high speed steel W6Mo5Cr4V2 by plasma cladding technology. Metallurgical bonding coating with enough bonding strength was obtained after optimizing the cladding parameters. The clad coating composition, microstructure, microhardness and thermal physical parameters were analyzed by X-ray diffractometer (XRD), optical microscopy (OPM), microhardness tester, laser thermal constant tester and dynamic thermo-mechanical analysis. The experimental results show that Ni60-SiC cermet coating can greatly improve the hardness of hot forging die surface, and reduce the thermal stress of hot forging die surface. Cladding coating has a very good performance of thermal properties.