The influence and defects of forging on the metal structure and properties of forgings

(1) The influence of forging on metal structure and properties

In forging production, in addition to ensuring the required shape and size of the forging, it must also meet the performance requirements of the part during use, which mainly include: strength index, plastic index, impact toughness, fatigue strength, initial fracture and Stress corrosion resistance, etc., for parts that work at high temperatures, there are instantaneous tensile properties, long-lasting properties, anti-stigmatization properties, and thermal fatigue properties for high-temperature parts.

The raw materials for forging are ingots, rolled materials, extruded materials and forging billets. The rolled material, extruded material and forging billet are respectively semi-finished products formed by rolling, extrusion and forging of ingots. In forging production, using reasonable technology and process parameters can improve the structure and performance of raw materials through the following aspects:

• 1. Break columnar crystals, improve macro segregation, change the as-cast structure into forged structure, and weld the internal pores under suitable temperature and stress conditions to increase the density of the material;
• 2. The ingot is forged to form a fiber structure, and further rolling, extrusion, and die forging are used to obtain a reasonable fiber direction distribution for the forging;
• 3. Control the size and uniformity of crystal grains;
• 4. Improve the distribution of the second phase (for example: alloy carbides in ledeburite steel);
• 5. Make the organization get deformation strengthening or deformation strengthening, etc.

Due to the improvement of the above-mentioned structure, the plasticity, impact toughness, fatigue strength and durability of forgings have also been improved, and then through the final heat treatment of the parts, a good combination of hardness, strength and plasticity required by the parts can be obtained. performance.

However, if the quality of the raw materials is poor or the forging process used is unreasonable, forging defects may occur, including surface defects, internal defects or unqualified performance.

(2) The influence of raw materials on the quality of forgings

Good quality of raw materials is a prerequisite for ensuring the quality of forgings. Defects in the raw materials will affect the forming process of the forgings and the final quality of the forgings.

If the chemical elements of the raw materials exceed the specified range or the content of impurity elements is too high, it will have a greater impact on the forming and quality of the forging. Hot brittleness appears. In order to obtain intrinsically fine-grained steel, the residual aluminum content in the steel needs to be controlled within a certain range, for example, A1 acid 0.02%~0.04% (mass fraction). If the content is too small, it will not play the role of controlling the large grain size, and it is often easy to make the essential grain size of the forging unqualified; if the aluminum content is too much, it is easy to form wood grain-like fractures and tears under the condition of forming the fibrous structure during pressure processing. Scar-like fractures, etc. For another example, in austenitic stainless steel, the more the content of n, Si, Al, and Mo, the more the elementary phase, the easier it is to form band-shaped cracks during forging, and make the parts magnetic.

If there are defects such as shrunken tube residue, subcutaneous blistering, severe carbide segregation, and coarse non-metallic inclusions (slag inclusions) in the raw material, cracks are likely to occur in the forging during forging. Defects such as dendrites, severe porosity, non-metallic inclusions, white spots, oxide film, segregation zone, and dissimilar metal mixing in the raw materials can easily cause the performance of forgings to decrease. Surface cracks, folds, scars, coarse crystal rings, etc. of the raw materials are likely to cause surface cracks on the forgings.

(3) The influence of forging process on the quality of forgings

The forging process generally consists of the following processes, namely 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 furnace loading temperature, heating temperature, heating speed, holding time, furnace gas composition, etc. If the heating is improper, for example, the heating temperature is too high and the heating time is too long, it will cause defects such as decarburization, overheating, and overburning.

For bad materials 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 will often be uneven, causing thermal stress and cracking of the forging blank.

Forging forming process includes deformation method, deformation degree, deformation temperature, deformation speed, stress state, tool and die conditions and lubrication conditions, etc. If the forming process is improper, it may cause coarse grains, uneven grains, various cracks, and folds. Overlap, through-flow, eddy current, as-cast structure residue, etc.

During the cooling process after forging, if the process is improper, it may cause cooling cracks, white spots, network carbides, etc.

(4) The influence of the forging structure on the structure and performance after the final heat treatment

Austenitic and ferritic heat-resistant stainless steels, high-temperature alloys, aluminum alloys, magnesium alloys, etc., during the heating and cooling process, materials without allotropic transformation, as well as some copper alloys and titanium alloys, are produced during the forging process The organization defect cannot be improved by heat treatment.

In the heating and cooling process, the materials with allotrope transformation, such as structural steel and martensitic stainless steel, etc., due to some structural defects caused by improper forging process or some defects left by the original material, the forgings after heat treatment Quality has a big impact. Examples are as follows:

• 1. The structural defects of some forgings can be improved during the post-forging heat treatment, and satisfactory structure and performance can still be obtained after the final heat treatment of the forgings. For example, in general overheated structural steel forgings, the coarse grains and Widmanstatten structure, hypereutectoid steel and bearing steel due to improper cooling caused by slight network carbides, etc.
• 2. The structural defects of some forgings are difficult to eliminate with normal heat treatment, and it is necessary to use high-temperature normalizing, repeated normalizing, low-temperature decomposition, high-temperature diffusion annealing and other measures to be improved.
• 3. The structural defects of some forgings cannot be eliminated by the general heat treatment process. As a result, the performance of the forgings after the final heat treatment is reduced or even unqualified. For example, severe stone fractures and edge fractures, overburning, ferrite bands in stainless steel, carbide meshes and bands in ledeburite high-alloy tool steels, etc.
• 4. The structural defects of some forgings will further develop during the final heat treatment, and even cause cracking. For example, if the coarse-grained structure in alloy structural steel forgings is not improved during heat treatment after forging, it will often cause coarse martensite and unqualified properties after carbon, nitriding and quenching; coarse band-like carbides in high-speed steel , Quenching often causes cracking.

Different forming methods have different stress and strain characteristics due to their different stress conditions, so the main defects that may be produced are also different. For example, the main defect of the blank upsetting is the longitudinal or 45° cracks on the side surface, and the as-cast structure is often left at the upper and lower ends of the ingot upsetting; the main defect of the rectangular cross-section blank is the transverse cracks on the surface. And corner cracks, internal diagonal cracks and transverse cracks; the main defects in open die forging are dissatisfaction, folding and misalignment.

Different types of materials, due to their different compositions and structures, have different structural changes and mechanical behaviors during heating, forging and cooling. Therefore, improper forging processes may cause defects that have their particularities. For example, the defects of ledeburite high-alloy tool steel forgings are mainly coarse carbide particles, uneven distribution and cracks. The defects of high-temperature alloy forgings are mainly coarse grains and cracks; if the defects of austenitic stainless steel forgings are intergranular chromium depletion, Decreased resistance to intergranular corrosion, ferrite banded structure and cracks, etc.; the defects of aluminum alloy forgings are mainly coarse grains, folding, eddy currents, and flow through.