Improvement steps:
(1) Require the rolling mill to improve the quality of steel. The rolling mill adopts electroslag remelting, increases the crystallization rate of molten steel, increases the forging ratio, improves the finishing temperature and diffusion annealing to improve or avoid the formation of band structure. .
(2) Strengthen management and improve the system
1 Start from the source, strictly put the raw materials into the factory, sign the steel technology agreement with the steel mill, and require not only the strip structure is less than grade 3, but also the chemical composition, looseness and segregation of the purchased steel. Specific requirements for non-metallic inclusions, grain size, mechanical properties and surface roughness. For the inspection of the strip structure and surface roughness of the incoming steel, it is required that the steel with a band structure of less than grade 3 and having a satisfactory surface roughness (including other indicators also qualified) can enter the factory.
2 Before the steel is put into production, a low-fold thermal acid etching test should be carried out to prevent the flow of the original cracked forgings into the next process.
3 For the fastener bar with poor surface roughness, rough the car before heat treatment to improve the surface quality and eliminate the stress concentration source of subsequent quenching and tempering treatment.
After adopting the above improvement measures, no similar problems occurred, the production was carried out normally, the crack parts were eliminated, and the product quality was effectively controlled.
Analyze the cracking of vehicle fasteners and preventive precautions:
1. Technical requirements for fasteners
1.1 Materials
1.2 Technical requirements
2, heat treatment process and equipment
(1) The fastener bar material is quenched and heated by RJJ210529T type well gas carburizing furnace. The cleaning and degreasing uses a self-made 60 kW cylindrical cleaning tank. The tempering adopts the DL150 type well type tempering furnace.
(2) The quenching and tempering process route is 930 °C heating quenching → 20 to 30 min oil tank quenching → (70 ~ 80) ° C × 5 min cleaning → 650 ° C tempering → inspection.
3. Inspection methods and results
3.1 Macro analysis
The cracks of the failed parts are longitudinal cracks, and the surface cracks are 1-3, and most of the cracks are deep to the center.
3.2 Chemical composition testing
Arbitrarily find a cracked piece for analysis, and the test results of the main chemical components meet the standard.
3.3 hardness determination
In the middle of the axial direction of the crack member, it was machined into a plane with a width of 60 mm and a depth of 2 mm. The Brinell hardness was measured by a HB23000 Brinell hardness tester, and the test results were 310HB, 315HB, and 312HB.
3.4 Metallographic analysis
The cracked bar was cut transversely and the crack was found to be 10 to 30 mm deep from the surface to the center. Metallographic analysis was performed from the outer part to the core near the crack. The result was: a more uniform distribution of tempered sorbite, grade 3 , qualified; grain size is 5, qualified.
The cracked bar was cut into a test piece and slowly annealed at 900 ° C for 1 h. As a result, the crack cracked and spread along the ferrite band in the banded structure, and the crack was parallel to the direction of the banded structure. The metallographic analysis near the crack is that the banded structure is extremely coarse and larger than grade 4.
3.5 Crack fracture morphology observation
The cracked test piece was cut along the crack surface by wire cutting, and the fracture was examined. The observation result was that there was no oxidation color on the crack surface, and there was no decarburization around the crack, indicating that the crack was generated during quenching and subsequently. The crack surface is smoother and finer than the gray and rough state, so it can be judged that the quenching heating temperature is too high or the original structure is coarse.
4. Analysis and discussion of results
(1) According to the chemical composition of the tested failure parts, the material of the steam turbine fastener bar material conforms to the national standard GB/T122121992 "Chemical composition and mechanical properties of ferritic martensitic heat-resistant steel", so the material is correct.
(2) According to the various morphological features revealed by the comprehensive analysis of the cracked parts, it can be judged that the crack is generated not because the quenching heating temperature is too high or the heating time is too long or the original structure is coarse, which is generated when the workpiece is quenched and subsequently.
(3) It is pointed out from the literature that the band structure in low-carbon alloy steel is formed during the rolling process of steel, which is formed by stacking the pre-eutectoid ferrite-based belt and the pearlite-based belt. As-cast structures are defective structures that often appear in steel.
Since the molten steel is selectively crystallized during the crystallization of the ingot, a dendritic structure in which the chemical composition is unevenly distributed is formed. The coarse dendrites in the ingot are elongated in the direction of deformation during rolling and gradually conform to the direction of deformation, so that the depleted bands of carbon and alloying elements are alternately stacked with each other.
In slow cooling, the pro-eutectoid ferrite is precipitated in the carbon and alloy element depletion zone, and the excess carbon is discharged into the enriched zone on both sides, eventually forming a ferrite-based zone; carbon and alloy. In the enriched zone of the element, the supercooled austenite has higher stability and then transforms into a pearlite-based band, thus forming a ferrite-based band and a pearlite-based band alternating with each other. Banded tissue. Therefore, the formation of the banded structure is due to the presence of more severe segregation in the as-cast microstructure. After hot rolling, the segregation zone will form a banded structure. The more severe the segregation of the components, the more severe the banded structure formed. The band segregation of alloying elements is a necessary condition for the formation of the banded pearlite 2 ferrite structure, and the banding segregation of such alloying elements is not easily eliminated.
The appearance of banded structure increases the tendency of the workpiece to be distorted and cracked. From this, it can be judged that the main cause of the axial crack of the cracked bar is due to the severe overshoot of the banded structure.
Because the microstructure of the adjacent bands of the banded structure is different, their properties are different. Under the action of external forces (such as thermal stress and tissue stress generated during heat treatment), the tape with low performance is easily exposed, and between the strong and weak bands. Stress concentration occurs, resulting in a decrease in overall mechanical properties and significant anisotropy. This results in a material whose transverse rupture strength is much lower than the longitudinal rupture strength. During the heat treatment, the stress of the microstructure and the thermal stress in the segregation microdomains where the components are not uniform will cause stress concentration, which eventually causes the bar to crack along the weak zone during the heat treatment quenching process.
Through analysis, 2Cr12NiMoWV is a highly hardenable martensitic steel, and the microstructure stress is large during the heat treatment, which also increases the crack generation. The site investigation also found that the surface of the cracked bar was rough, and there were fine furrows along the axial surface. This is caused by the bar in the forging process. These places are prone to stress concentration and are weak in inducing quenching cracks. Link.
The Intelligent Energy Storage System
The Intelligent Energy Storage System,Large Photovoltaic Grid-Connected Power,Grid-Connected Power Generation System,Photovoltaic Grid-Connected Power Generation System
Fuzhou Mei Li Cheng Imp&Exp Co., Ltd , https://www.mlc-solar.com