Effect of the hottest forging on the microstructur

Influence of forging on metal structure and properties

defects of forgings include surface defects and internal defects. Some forging defects will affect the processing quality of subsequent processes, while others will seriously affect the performance of forgings, reduce the service life of finished products, and even endanger safety. Therefore, in order to improve the quality of forgings and avoid the occurrence of forging defects, corresponding technological countermeasures should be taken, and the quality control that may lead to inconsistent test results in the whole production process should be strengthened. This chapter briefly introduces three aspects: the influence of forging on metal structure and properties and forging defects; Contents and methods of forging quality inspection; The general process of forging quality analysis

(I) influence of forging on metal structure and properties

in forging production, in addition to ensuring the shape and size required by forgings, it must also meet the performance requirements put forward by parts in the process of use, mainly including: strength pointer, plastic pointer, impact toughness, fatigue strength, fracture toughness and stress corrosion resistance. For parts working at high temperature, there are also high-temperature instantaneous tensile properties, durability Creep resistance and thermal fatigue properties. The raw materials used for forging are ingots, rolled stock, extruded stock and forging stock. Rolling stock, extruding stock and forging stock are semi-finished products formed by rolling, extruding and forging ingots respectively. In forging production, adopting reasonable process and process parameters can improve the structure and properties of raw materials through the following aspects: (1) breaking columnar crystals, improving macro segregation, changing the as cast structure into as forged structure, and welding internal pores under appropriate temperature and stress conditions to improve the density of materials; (2) The ingot is forged to form a fiber structure, and further through rolling, extrusion and die forging, the forgings get a reasonable fiber direction distribution; (3) Control the grain size and uniformity; (4) Improve the distribution of the second phase (such as alloy carbide in ledeburite steel); (5) Make the structure get deformation strengthening or deformation transformation strengthening, etc. Due to the improvement of the above structure, the plasticity, impact toughness, fatigue strength and durability of forgings have also been improved. Then through the final heat treatment of the parts, the hard freight rate required by the parts can be gradually stabilized and have good comprehensive properties such as falling trend, strength and plasticity. However, if the quality of raw materials is poor or the forging process used is unreasonable, forging defects may occur, including surface defects, internal defects or unqualified performance

(II) influence of raw materials on the quality of forgings

the good quality of raw materials is the prerequisite to ensure the quality of forgings. If the raw materials have defects, it will affect the forming process of forgings and the final quality of forgings. If the chemical elements of raw materials exceed the specified range or the content of impurity elements is too high, it will have a great impact on the forming and quality of forgings. For example, s, B, Cu, Sn and other elements are easy to form low melting point phases, making forgings prone to hot embrittlement. In order to obtain essentially fine grain steel, the residual aluminum content in the steel needs to be controlled within a certain range, such as Al acid 0.02% - 0.04% (mass fraction). If the content is too small, it can not control the grain growth, and it is often easy to make the essential grain size of forgings unqualified; With excessive aluminum content, it is easy to form wood grain fracture and tear mark fracture under the condition of forming fiber structure during pressure processing. For another example, in 1Cr18Ni9Ti austenitic stainless steel, the more content of Ti, Si, Al, Mo, the more ferrite phase, the easier it is to form banded cracks during forging, and make the parts magnetic. For example, defects such as tube shrinkage residue, subcutaneous blistering, severe carbide segregation, coarse non-metallic inclusions (slag inclusions) and so on exist in raw materials, which are easy to cause cracks in forgings during forging. Defects such as dendrites, serious porosity, non-metallic inclusions, white spots, oxide films, segregation bands and mixed dissimilar metals in raw materials are easy to cause the performance of forgings to decline. Surface cracks, folds, scabs, coarse-grained rings, etc. of raw materials are easy to cause surface cracks of forgings

(III) influence of forging process on forging quality

forging process generally consists of the following processes, namely blanking, heating, forming, post forging cooling, pickling and post forging heat treatment. A series of forging defects may occur in the forging process if the process is improper. The heating process includes charging temperature, heating temperature, heating speed, holding time, furnace gas composition, etc. If the heating is improper, such as the heating temperature is too high and the heating time is too long, it will cause decarburization, overheating, overburning and other defects. For billets with large section size, poor thermal conductivity and low plasticity, if the heating speed is too fast and the holding time is too short, the temperature distribution is often uneven, causing thermal stress and cracking of billets. The forging forming process includes deformation mode, deformation degree, deformation temperature, deformation speed, stress state, tool and die conditions and lubrication conditions. If the forming process is improper, coarse grains, uneven grains, various cracks and folds may be caused. Cold current, eddy current, as cast microstructure residue, etc. In the cooling process after forging, if the process is not proper, it may cause cooling cracks, white spots, like carbides, etc

(IV) effect of forging microstructure on Microstructure and properties after final heat treatment

austenitic and ferritic heat-resistant stainless steel Gao Nanzhuang always plays "Enterprises develop warm alloys, aluminum alloys, magnesium alloys, etc. in the heating and cooling process, materials without isomorphic transformation, as well as some copper alloys and titanium alloys, the structural defects produced in the forging process cannot be improved by heat treatment. Materials with isomorphic transformation in the heating and cooling process, such as structural steel and martensitic stainless steel, have some structural defects or raw materials left over due to improper forging process Some defects of have a great influence on the quality of forgings after heat treatment. Examples are as follows:

(1) the structural defects of some forgings can be improved during post forging heat treatment, and the forgings can still obtain satisfactory microstructure and properties after final heat treatment. For example, coarse grain and widmanstatten structure in general overheated structural steel forgings, slight carbides caused by improper cooling of hypereutectoid steel and bearing steel, etc

(2) the structural defects of some forgings are difficult to be eliminated by normal heat treatment. Measures such as high-temperature normalizing, repeated normalizing, low-temperature decomposition, high-temperature diffusion annealing and so on are required. The main composition of the equipment is: can it be improved. For example, low magnification coarse grain, twin carbide of 9Cr18 stainless steel, etc

(3) the structural defects of some forgings cannot be eliminated by general heat treatment process, resulting in the decline of the properties of forgings after final heat treatment, or even unqualified. For example, serious stone fracture and edge fracture, overburning, ferrite band in stainless steel, carbide and band in ledeburite high alloy tool steel, etc

(4) the structural defects of some forgings will further develop and even cause cracking during the final heat treatment. For example, if the coarse-grained structure in alloy structural steel forgings is not improved during post forging heat treatment, the coarseness and unqualified performance of martensite needles are often caused after carbon, nitrocarburizing and quenching; The coarse banded carbides in high-speed steel often cause cracking after quenching. The common defects in the forging process and their causes will be introduced in detail in the second chapter. It should be pointed out that the common defects in various forming methods and the main defects of forgings of various materials have their own rules. Different forming methods, due to their different stress conditions and stress-strain characteristics, may produce different main defects. For example, the main defects of blank upsetting are the longitudinal or 45 ° cracks on the side surface, and the residual as cast structure at the upper and lower ends of ingot upsetting; The main defects of rectangular section blank during drawing are transverse crack and angular crack on the surface, diagonal crack and transverse crack inside; The main defects of open die forging are insufficient filling, folding and dislocation. The common defects in the main forming processes will be introduced in detail in Chapter 4. Due to the different composition and structure of different kinds of materials, their structural changes and mechanical behavior are also different in the process of heating, forging and cooling. Therefore, the possible defects caused by improper forging process also have their particularity. For example, the defects of ledeburite high alloy tool steel forgings are mainly coarse carbide particles, uneven distribution and cracks, while the defects of Superalloy Forgings are mainly coarse grains and cracks; The defects of austenitic stainless steel forgings are mainly intergranular chromium deficiency, decreased intergranular corrosion resistance, ferrite banded structure and cracks, etc; The defects of aluminum alloy forgings are mainly coarse grain, folding, eddy current, through flow and so on