Effect of forging on microstructure and properties of metal
In forging production, in addition to ensuring the required shape and size of forged flanges, it must also meet the performance requirements of parts in the process of use, which mainly include: strength index, plasticity index, impact toughness, fatigue strength and initial fracture degree For parts working at high temperature, there are also high temperature instantaneous tensile property, endurance property, anti creep property and thermal fatigue property.
The raw materials for forged flanges are ingot, rolling stock, extrusion stock and forging stock. Rolling stock, extrusion stock and forging stock are semi-finished products formed by rolling, extrusion and forging respectively. In forging production, the structure and properties of raw materials can be improved by adopting reasonable process and process parameters:
- 1. The results show that the columnar crystal is broken, the macrosegregation is improved, the as cast structure is changed into the forged structure, and the internal pores are welded under the appropriate temperature and stress conditions to improve the density of the material;
- 2. The fiber structure of ingot is formed by forging, and reasonable fiber direction distribution is obtained by rolling, extrusion and die forging;
- 3. Controlling the size and uniformity of grains;
- 4. Improve the distribution of the second phase (such as alloy carbide in ledeburite steel);
- 5. It can strengthen the structure by deformation.
Because of the improvement of the above-mentioned structure, the plasticity, impact toughness, fatigue strength and endurance properties of the forging are also improved, and then through the final heat treatment of the parts, the good comprehensive properties such as hardness, strength and plasticity required by the parts can be obtained.
However, if the quality of raw materials is poor or the forging process is unreasonable, the forging defects may occur, including surface defects, internal defects or unqualified performance.
Influence of raw materials on forging quality
Good quality of raw materials is a prerequisite to ensure the quality of forged flanges. If there are defects in raw materials, the forming process and final quality of forged flanges will be affected.
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 forged flanges, for example, s, B, Cu, Sn and other elements are easy to form low melting point phase, which makes forgings prone to hot embrittlement. In order to obtain essentially fine grain steel, the residual aluminum content in the steel should be controlled within a certain range, such as 0.02% – 0.04% (mass fraction) of A1 acid. If the aluminum content is too small, it can’t control the grain size, so it is easy to make the essential grain size of forged flanges unqualified; if the aluminum content is too high, 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 austenitic stainless steel, the more N, Si, Al and Mo are contained, the more ferrite phases are, the easier it is to form banded cracks during forging and make the parts magnetic.
If there are some defects in the raw materials, such as residual shrinkage, blistering under the skin, serious carbide segregation, coarse non-metallic inclusions (slag) and so on, the forging is easy to produce cracks. The defects in raw materials, such as dendrite, serious porosity, non-metallic inclusion, white spot, oxide film, segregation band and mixture of different metals, are easy to cause the performance degradation of forgings. The surface cracks of forgings are easily caused by surface cracks, folds, scabs and coarse-grained rings of raw materials.
Influence of forging process on forging quality
The forging process generally consists of the following processes: blanking, heating, forming, cooling after forging, pickling and heat treatment after forging. If the forging process is improper, a series of forging defects may occur.
The heating process includes charging temperature, heating temperature, heating speed, holding time, furnace gas composition, etc. Improper heating, such as too high heating temperature and too long heating time, will cause decarbonization, overheating, overburning and other defects.
If the heating speed is too fast and the holding time is too short, the temperature distribution is often uneven, resulting in thermal stress and cracking of forging blank.
Forging forming process includes deformation mode, deformation degree, deformation temperature, deformation speed, stress state, die condition and lubrication condition. If the forming process is not proper, coarse grains, uneven grains, various cracks, folding, flow through, eddy current and residual as cast structure may be caused.
During the cooling process after forging, if the process is not proper, it may cause cooling cracks, white spots, network carbides and so on.
Effect of forging microstructure on Microstructure and properties after final heat treatment
Austenitic and ferritic heat-resistant stainless steel, superalloy, aluminum alloy, magnesium alloy and other materials without isomerism transformation during heating and cooling, as well as some copper alloy and titanium alloy, can not be improved by heat treatment.
In the process of heating and cooling, some structural defects caused by improper forging process or some defects left by raw materials have great influence on the quality of forged flanges after heat treatment. Examples are as follows:
- 1. The microstructure defects of some forged flanges can be improved during post forging heat treatment, and satisfactory microstructure and properties can be obtained after final heat treatment. For example, coarse grain and widmanstatten structure in general overheated structural steel forgings, slight network carbide in hypereutectoid steel and bearing steel due to improper cooling, etc.
- 2. The structure defects of some forged flanges are difficult to be eliminated by normal heat treatment, which can be improved by high temperature normalizing, repeated normalizing, low temperature decomposition, high temperature diffusion annealing and other measures.
- 3. The structure defects of some forgings can’t be eliminated by general heat treatment process, resulting in the performance degradation or even disqualification of forgings after final heat treatment. For example, severe stone fracture and edge fracture, overburning, ferrite band in stainless steel, carbide net and band in ledeburite high alloy tool steel, etc.
- 4. The structure defects of some forgings will be further developed in the final heat treatment, and even cause cracking. For example, if the coarse-grained structure of alloy structural steel forgings is not improved during post forging heat treatment, the coarse martensite and unqualified properties are often caused after carbonitriding and quenching; the coarse banded carbide in high-speed steel often causes cracking after quenching.
Different forming methods have different stress and strain characteristics, so the main defects may be different. For example, the main defects during upsetting are longitudinal or 45 ° cracks on the side surface, and only the upper and lower ends of the ingot usually have as cast structure; the main defects during drawing rectangular section billet are transverse cracks and corner cracks on the surface, diagonal cracks and transverse cracks inside; the main defects during open die forging are underfill, folding and dislocation.
Different kinds of materials, due to their different composition and microstructure, have different microstructure changes and mechanical behaviors in the process of heating, forging and cooling. Therefore, when forging process is not appropriate, the possible defects also have their particularity. For example, the main defects of ledeburite high alloy tool steel forgings are coarse carbide particles, uneven distribution and cracks; the main defects of high temperature alloy forgings are coarse grains and cracks; the main defects of austenitic stainless steel forged flanges are poor intergranular chromium, reduced intergranular corrosion resistance, ferrite banded structure and cracks; the main defects of aluminum alloy forgings are coarse grains, folding, eddy current, etc And so on.
Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)