สมบัติทางโลหะวิทยาและสมบัติทางกลของการเชื่อมพอกแข็งเหล็กกล้า 3.5% โครเมียม

Abstract

Coal crusher made by 3.5% Cr steel is an important piece of equipment in the production of coal, but its abrasive wear remains a troublesome problem. A worn coal crusher damaged by abrasive wear is usually repaired by hardfacing weld deposit to prolong its lifespan. Currently, hardfacing using arc welding with and without added metal powder is gaining interest. Therefore, this research aimed to study the hardfacing of 3.5% chromium steel. The welding experiment was divided into 2 parts. The first part of welding experiment was the hardfacing by shielded metal arc welding process, flux cored arc welding and submerged arc welding using the martensitic steel wire electrode as a filler metal. The second part of welding experiment was the hardfacing by submerged arc welding using a low carbon steel wire electrode with added metal powder. Then, the welded samples were studied macroscopic and microscopic structure, percent dilution, chemical composition, crystalline structures, and worn surface. Mechanical properties were assessed by hardness and abrasive wear test. The results of the first part of welding experiment showed that the cross-section of welding samples was perfect. The deposited layers using lower heat input resulted in lower welding dilution than those using higher heat input. Submerged arc welding using alternating current polarity produced shallower dilution compared to using direct current positive polarity. Heat affected zone microstructure of the single-layer hardfacing showed martensite structure, whereas a tempered martensite structure was found on that of the three-layer hardfacing. The microstructure of buffer layer was austenitic structure and delta ferrite at grain boundaries. The microstructure of hardfacing layer was martensitic and austenitic structure. The first hardfacing layer was mixed with austenite buffer layer resulting in high amount of austenite structure. Therefore, hardness and wear resistance of the first hardfacing layer were less than those of the third hardfacing layer. The worn surfaces of the hardfacing layer in this experimental part found that the dominant wear mechanisms are micro-cutting and micro-ploughing. In accordance with the first part of experimental welding results, the flux cored arc weld deposit showed the best wear resistance in both single-layer and three-layer hardfacing. The results of the second part of welding experiment revealed that most of the hardfacing layer with added powder contained cracks. Increasing the amount of added powder increased the size and the thickness of hardfacing layer and decreased the percent dilution of welding as well. The chemical composition of the first hardfacing layer obtained from both the dilution by the buffer layers and the added metal powder resulted in a martensitic microstructure as the main structure, which gave a good hardness and wear resistance. For the third hardfacing layer, the microstructure of hardfacing layer depended primarily on the type of the added powder. The results showed that the hardfacing with ferro chromium powder resulted in the martensite structure. The hardfacing with ferro-carbon powder gave the pearlite and ferrite structure. The hardfacing with ferro-carbon and chromium powder presented the martensite structure which had the highest hardness. The hardfacing with added powder containing carbon, chromium, manganese and nickel as the main compound revealed the martensite and austenite structure. The worn surfaces of the hardfacing layer in this experimental part found that the prominent wear mechanisms are micro- cutting and micro-fracture. The second part of experimental welding results proved that the three- layer hardfacing using a submerged arc welding with added ferro-carbon and chromium powder was the most superior wear resistance among other conditions.