The reason of the change of the concentration of the composite wear-resistant steel plate
Release time：2018-6-28 18:33:09 Number of clicks：1376
The wear-resistant composite plate takes Ca(NO3)2 and NH4H2PO4 as the main electrolytic salts, and adds Zn(NO3)2 into the electrolyte. The zn-doped hydroxyapatite (HAP) coating was successfully prepared. By controlling the content of added Zn elements, the effect of Zn on the crystallinity, microstructure, phase composition and binding force between coating and titanium matrix of hydroxyapatite coating was studied and analyzed.
Based on metallurgical roll wear resistant composite steel plate surface submerged arc surfacing layer as the research object, using a stered microscope, scanning electron microscopy (sem) observed the wear-resistant composite plate welding layer crack and crack fracture macroscopic morphology, microscopic morphology. Hardness of surfacing layer, transition layer and substrate was measured by rockwell hardness tester. Metallographic microscopy was used to observe the metallographic structure of surfacing layer, transition layer and matrix. The residual austenite volume of surfacing layer was determined by XRD. The chemical composition of wear-resistant composite steel surfacing layer, transition layer and matrix material was determined by means of emission spectrometer. X - ray energy spectrometer was used to determine the chemical composition of micro-area on fracture surface. The above method is used to determine roll failure characteristics and failure reasons.
By changing the concentration of Ca(NO3)2 and NH4H2PO4, the effect of electrolyte concentration on electrochemical deposition of calcium phosphate coating products was studied.
The results showed that: as the electrolyte concentration increased, the coating product phase by HAP gradually to eight calcium phosphate (OCP) composition among transformation, at the same time the microstructure observed from the nanoscale rules hexagonal HAP crystals gradually transformed into micron grade ribbon bar OCP process of crystal.
According to the surfacing environment of the roll, the temperature field during the surfacing and cooling process of the roll weld and its adjacent area is simulated by thermal-structural coupling method, and the axial, circumferential and radial thermal stress fields are analyzed. The thermal stress field of surfacing layer under different energy input of welding line is compared.
The main conclusions of this paper are as follows:
(1) short cracks and microcracks along the crystal were observed on the surfacing layer and in the longitudinal section of the roll abrasion-resistant composite steel plate, while no cracks were observed in the surfacing layer and the parent material of the transverse section. The short crack is a crystalline crack (the length is 1 ~ 10mm), which is mainly transverse to the center of the weld. The microcrack along the crystal is a reheat crack, which is located in the coarse crystal area near the crack area.
(2) the overall P content of the surfacing layer (0.025%) exceeds the technical requirements and the excessive content of P(0.05-0.06%) and S(0.05-0.12%) on the surface of dendritic crystals is the component factor for the formation of crystalline cracks in the surfacing layer of composite abrasion-resistant plates. The surfacing layer material contains high content of Mo, V and other carbide forming elements, as well as grain boundary weakening elements P(0.04%) and S(0.03%), which are the component factors for the formation of reheat cracks in the surfacing layer.
(3) the simulated axial thermal stress peak area is located in the center of the roll weld, and the circumferential and radial thermal stress peak area is located in the roll thermal influence area. The axial thermal stress (242 mpa, 1100 ℃) than circumferential thermal stress (171 mpa, 1100 ℃) and radial thermal stress (48 mpa, 1100 ℃). Axial thermal stress is large, and the occurrence of peak thermal stress in the center of roll weld is the mechanical factor of transverse cracking of surfacing layer.
(4) the simulated thermal stress decreases with the decrease of the width of the weld bead, that is, the thermal stress decreases with the decrease of the energy input of the weld bead. Reducing the energy input of welding line can avoid overheat stress of overlaying welding.
The results show that with the increase of Zn content in the electrolyte, the crystallinity of HAP crystals deposited gradually decreases, and the average size of the crystals gradually decreases. When the content of Zn reaches 25mol%(Zn/(Zn+Ca), no zn-hap crystals can be obtained by deposition. At the same time, with the increase of Zn content, the regular hexagonal microstructure characteristics of coating gradually disappeared, and the binding force between coating and titanium matrix gradually increased.
In addition, this paper also analyzes the internal cause of this change caused by the change of the concentration of the composite wear-resistant steel plate from the perspective of thermodynamics. By controlling the deposition time of different electrolyte concentration, the formation mechanism of HAP and OCP by electrochemical deposition was preliminarily revealed from the microscopic morphology.
It is proposed in the paper that: at the initial stage of deposition at low electrolyte concentration (c(Ca2+)=1.2 * 10-3mol/L), thin sheet OCP crystals are obtained. As deposition progresses, they are transformed into HAP crystals by solid-solid transformation. When the electrolyte concentration within a certain range (c (Ca2 +) = 2.4 * 10-3 mol/L ~ 3.6 * 10-3 mol/L), deposition was lower crystallinity of HAP crystals, it is early by deposition of nano particles in laminar OCP before directional adhesion formation on the surface of the body, belongs to the classical nucleation; When the concentration of electrolyte is high (c(Ca2+)4.8 * 10-3mol/L), OCP crystals are formed through the laminar reaction of classical nucleation theory.
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