Research Of Middle Temperature Chromizing On Carbon Steel With Double Glow Plasma Surface Alloying Process And Carbon Migration In The Chromized Layer

Die and mould are the necessary tools in plastic process. Middle and high carbon steel is the cheap material of making die and mould, which often become invalid because of low wear resistance, oxidation resistance and corrosion resistance on the surface, hence the extreme importance of the surface hardening technique, but quite many problems are hindering the application of now existing techniques such as easy flaking of hardened layer, high temperature of heat treatment and pollution, etc.In the past twenty years, double glow plasma surface alloying process has made considerable progress, but comprehensive and deep researches on the surface of middle and high carbon steel are still not conducted. In particular, when the permeated element is the one that can form high carbon compound, the function, the influence and the use of C element in the substrate will become very important. What are the influences of carbon in the substrate on the formation of chromized layer when the surface of high carbon alloy like middle and high carbon steel is chromized. Conversely, what are the influences of chromium on the distribution of carbon in the substrate. Such seemingly simple questions are, in fact, of important theoretically instructional functions to the development and application of this technique.In the light of the engineering practical problems such as the surfacehardening of carbon steel, the improvement of wear resistance, corrosion resistance and oxidation resistance at a high temperature, the paper takes chromizing as the means of surface hardening. In particular, for some workpieces requiring high surface wear resistance but rather low substrate proprieties, it is a thought of application value to use double glow plasma surface alloying process to form high hardness layer on the surface, however, double glow plasma surface alloying process hasn't been researched on this respect.Middle temperature (800~950°C ) chromizing on carbon steel with double glow plasma surface alloying process through the strengthening of alloying effect after iron bombardment can decrease workpiece deformation without destroying substrate organization and proprieties and is of great value for increasing the practicality of double glow plasma surface alloying process and broadening its application field.In the light of the above questions, this paper chose middle temperature chromizing on carbon steel with double glow plasma surface alloying process and its effects on the substrate as its research subject, and started the experimental study and theoretical analysis around the application basis problems of double glow plasma surface chromizing hardening technique in middle and high carbon steel surface at a temperature from 880°C to 900°C. This paper chooses 45,55,T8,T10 steel as experimental materials and adopts double glow plasma surface alloying process to conduct the basic and applied study of surface alloying.Diffusing atom theory holds that the temperature of workpiece have a great influence on the growth of alloyed layers and matrix properties, therefore, it seems extremely important to measure the temperature of workpiece accurately. In the process of double glow plasma surface alloying, workpieces are placed under negative bias in the plasma in the vacuum room, so there are a lot of difficulties to be solved to measure the temperature accurately. The paper firstly does some experiments and research on temperature measurement of plasma surface alloying processand reaches some laws that the workpiece real temperature can be accurately measured with the photoelectric pyrometer.The method of measuring workpiece temperature with the WDL-31 photoelectric pyrometer is simpler than that with the thermocouple, but the photoelectric pyrometer should be calibrated according to the measurement conditions before used.At the stable stage of double glow plasma chromizing on the carbon steel surface by using conventional process, the photoelectric pyrometer needs to be used to measure different workpiece temperature between 800 °C and 1000°C with different e values respectively.Only in this way can indicating value be identical to workpiece real temperature.Gas pressure in the furnace greatly effects temperature measurement precision of the photoelectric pyrometer, in the alloying processes with different gas pressure, the photoelectric pyrometer should choose different e values to measure temperature .By adopting different processing parameters, different forms of chromized layers such as deposition layer, diffusion layer, alloying layer and so on are gained. The alloying layer which consists of pure chromium deposit layer, Cr-C compound layer and chromium-bearing ferrite layer is an often seen form of chromized layer in middle and high carbon steel surface.The paper uses the advantage that double glow plasma surface alloying process can adjust timely some process parameters, the process conditions that carbon steel surface alloying forms chromium diffusion layer and chromium deposit layer are gained through changing permeated metal element concentration on the surface of substrate. The paper conducts the study of controlling alloying layer structure by controlling macroscopical process parameters.Aiming at getting chromized layer samples in which Cr-C compound layer is the thickest, while chromium deposit layer and substrate decarbonized layer is the thinnest, the optimal processing parameters ofdouble glow plasma surface chromizing process on carbon steel at a temperature from 880°C to 900°C is arrived at: polar distance 20mm, thesputtering voltage -800v, the discharge voltage -280v-----350v, gaspressure 45Pa, chromizing time 3h.The paper induces and sort out a lot of prophase experimental results. In the light of the characteristics of chromized layer prepared by double glow plasma surface alloying, quality evaluation index of chromized layer consisting of chromized layer integrity, wane degree, chromized layer thickness, chromium deposit thickness, Cr-C compound thickness, substrate decarbonized layer thickness and substrate grain growth degree is put forward. And quality evaluation index system is established. Quantitative assessment reference identifying the effects of chromizing on the carbon steel surface is offered.This paper systematically detects the components and microstructure of chromized layer on the carbon steel surface after double glow plasma surface chromizing process by adopting the optimal processing parameters. Fe, Cr, C concentration distribution along section in the chromized layer is measured through point-by-point microscanning with GDA750 glow discharge spectrum analysis (GDA) and KYKY-1000B scanning electron microscopy(SEM); the microstructure and micrograph in the chromized layer is observed and the thickness is measured with Axiovert25 micrograph analysis; phase analysis of the chromized layer is conducted with Rigaku D/max2500 X-ray diffractometer(XRD)oNo matter it is in the middle carbon steel or in the high carbon steel, Cr concentration in the chromized layer changes with depth, that is, gradually decreases from the surface to the center and thus form a kind of gradient alloy material, the highest Cr concentration of which can reach above 95% in the surface layer. The Cr concentration distribution curve is in the shape of a right half parabola with downward opening. The chromized layer thickness in middle carbon steel can reach above 45 u m, and that in high carbon steel can reach above 60 u m. Chromiumdeposition layer is composed of pure chromium mixed with little (Fe,Cr) 23C6 phase. Cr-C compound layer is mainly made up of (Fe,Cr) 23C6 phase and (Fe,Cr) 7C3 phase ,and form dense continuous distribution. Chromium-bearing ferrite layer is a microstructure made up of chromium-bearing pearlite and cementite.This paper systematically detects the properties of chromized layer on the carbon steel surface after double glow plasma surface chromizing process by adopting the optimal processing parameters, hardness distribution of the chromized layer is measured under lOg load with M-400-H1 micro-sclerometer; the binding force between the deposit layer and the Cr-C compound layer is measured with WS-97 scratch tester; the wear resistance improvement of the chromized layer is tested through abrasion test contrast with ML-10 abrasive wear tester; the oxidation resistant of the chromized carbon steels are investigated in air ai 500 °C and 750°C for lOOh, by comparing the cyclic oxidation behaviors of chromized samples and not chromized samples.After chromization, the highest hardness of the carbon steel surface appears in the Cr-C compound layer, the Vickers hardness of which comes up to HV1200-HV1420, and under the chromized layer appears poor carbon layer, but its hardness is still higher than substrate hardness. The deposition layer thinner than 6u m won't flake and influence the sound combination with Cr-C compound layer. Abrasion wear test shows that compared to conventional quenching and drawing process, wear resistance of the carbon steel surface after double glow plasma surface chromizing process can rise 5 to 6 times. Cyclic oxidation test in 500°C and 750°C for lOOh shows the oxidation resistance of the chromized carbon steels can be increased by a factor of 10.The alloying course is divided into four parts through the microscopic analysis of the physical double glow plasma alloying process on carbon steel surface. Through the curves showing the thickness change of Cr—C compound layer and deposit layer with chromizing time, X-raydiffraction pattern and chromium concentration distribution, the paper analyses the growth rule of alloying layer and put forward the growth mode that the chromium on the surface diffuses before it deposits .In the course of double glow plasma surface chromizing process, when the surface of carbon steel sample is strongly splashed, the substrate and the chromized layer might be decarbonized. The paper draws the process condition that whether or not the middle and high carbon steel substrate and the chromized layer are decarbonized. The paper gives theoretical verification to the above processing conditions, utilizing crystal defect formation theory.When carbon steel surface is dealt with double glow plasma chromizing, the diffusion of Cr causes the decrease of carbon chemical potential and carbon migration from substrate to chromized layer, which results in the occurrence of poor carbon region, which results in the occurrence of rich carbon region in the chromized layer, whose thickness is in direct ratio to the thickness of chromized layer and the Cr concentration in it. Through the curve showing the change of carbon concentration distribution in the chromized layer and the chromizing time, this paper describes the carbon migration laws in the course of diffusion of Cr into carbon steel substrate, analyses the carbon migration dynamics with thermodynamics knowledge, and reveals the interaction between the diffused element and the base material element.