Effects and Defects of Powder Forging on Forging Metal

At the end of the last century, a new metal plastic forming process, powder forging, was successfully developed abroad. It has successively forged automobile differential planetary gears and connecting rod forgings, and built the first powder forging production line. It is a competitive non-cutting metal processing method developed by organically combining traditional powder metallurgy technology with precision forging. Using metal powder as raw material, it is preformed and pressed in a protective atmosphere, heated and sintered as a forging blank, and forged in a press at one time to achieve precision die forging without flash, and obtain precision forgings with the same density and complex shape as ordinary die forging.

It not only has the advantages of good forming performance of powder metallurgy, but also plays the role of forging deformation in changing the organization and properties of metal materials, and has made new breakthroughs in powder metallurgy production and forging technology. It is a marginal professional discipline, especially suitable for mass production of high-strength and complex-shaped structural parts. Therefore, it has great promotion significance in various industrial fields.

Powder forging process flow
The general powder forging process is to form a powder with an appropriate formula and mixture in the same way as the manufacture of ordinary sintered parts, and make it into a low-density preform, which is used as a forging blank for hot forging after sintering. When the preform contains a lubricant, a lubricant removal process should be added before sintering. If it is cooled after sintering, it must be reheated before forging. Some degree of machining is usually required before heat treatment after forging. Throughout the process, except for short-term forging, all heating is carried out in an anti-oxidation protective atmosphere.

Compared with die forging, powder forging has the following advantages:

01 High material utilization rate, reaching more than 90%. The material utilization rate of die forging is only about 50%.

02 High mechanical properties. The material is uniform and non-anisotropic, with high strength, plasticity and impact toughness.

03 Forgings have high precision and smooth surfaces, and less and no cutting can be achieved.

(I) The influence of forging on metal structure and performance
In forging production, in addition to ensuring the required shape and size of the forging, the performance requirements of the parts during use must also be met, including: strength index, plasticity index, impact toughness, fatigue strength, fracture initial degree and stress corrosion resistance, etc. For parts working at high temperatures, there are also high-temperature instantaneous tensile properties, endurance performance, anti-deformation performance and thermal fatigue performance. The raw materials used for forging are ingots, rolled materials, extruded materials and forging billets. Rolled materials, extruded materials and forging billets are semi-finished products formed by rolling, extruding and forging of ingots. In forging production, the use of reasonable processes and process parameters can improve the organization and performance of raw materials in the following aspects:
1. Break columnar crystals, improve macro-segregation, change the cast structure into forged structure, and weld the internal pores under appropriate temperature and stress conditions to improve the density of the material;

2. The ingot is forged to form a fibrous structure, and the forging is further rolled, extruded, and die forged to obtain a reasonable fiber direction distribution;
3. Control the size and uniformity of the grains;
4. Improve the distribution of the second phase (for example: alloy carbides in ledeburite steel);
5. Make the organization deformed or strengthened. Due to the improvement of the above-mentioned organization, the plasticity, impact toughness, fatigue strength and endurance of the forging are also improved. Then, through the final heat treatment of the parts, the good comprehensive properties of the hardness, strength and plasticity required by the parts can be obtained. 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.

(II) The influence of raw materials on forging quality The good quality of raw materials is a prerequisite for ensuring the quality of forgings. If there are defects in the raw materials, it will affect the forming process of forgings and the final quality of 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 great impact on the forming and quality of forgings. For example, elements such as S, B, Cu, Sn are easy to form low melting point phases, making forgings prone to hot brittleness. In order to obtain intrinsic fine-grained steel, the residual aluminum content in the steel needs to be controlled within a certain range, such as 0.02%~0.04% (mass fraction) of Al. If the content is too little, it will not play a role in controlling the enlargement of grains, and it is easy to make the intrinsic grain size of forgings unqualified; if the aluminum content is too much, it is easy to form wood grain fractures, tear-like fractures, etc. under the condition of forming fibrous tissue during pressure processing. For example, in austenitic stainless steel, the more n, Si, Al, and Mo are contained, the more ferrite phases there are, and the easier it is to form band cracks during forging, and make the parts magnetic. If there are shrinkage residues, subcutaneous blistering, severe carbide segregation, coarse non-metallic inclusions (slag inclusions) and other defects in the raw materials, it is easy to cause cracks in the forgings during forging. Defects such as dendrites, severe looseness, non-metallic inclusions, white spots, oxide films, segregation bands and foreign metal mixing in the raw materials are easy to cause the performance of forgings to deteriorate. Surface cracks, folds, scars, coarse crystal rings, etc. of the raw materials are easy to cause surface cracks in forgings.

(III) The impact of forging process on the quality of forgings The forging process generally consists of the following procedures, namely, blanking, heating, forming, cooling after forging, pickling and heat treatment after forging. If the process is improper during the forging process, a series of forging defects may occur. 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 defects such as decarburization, overheating and overburning. For bad materials with large cross-section, 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 the forging blank. 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, it may cause coarse grains, uneven grains, various cracks, folding, permeation, eddy currents, and residual cast structure. During the cooling process after forging, if the process is improper, it may cause cooling cracks, white spots, network carbides, etc.

(IV) The influence of forging structure on the structure and properties after final heat treatment Austenite and ferrite heat-resistant stainless steel, high-temperature alloy, aluminum alloy, magnesium alloy, etc., materials without allotropic transformation during heating and cooling, as well as some copper alloys and titanium alloys, etc., the structural defects generated during the forging process cannot be improved by heat treatment. Materials that undergo allotropic transformations during heating and cooling, such as structural steel and martensitic stainless steel, have certain structural defects caused by improper forging processes or certain defects left over from the original material, which have a great impact on the quality of forgings after heat treatment. Here are some examples:
1. Some structural defects of forgings can be improved during post-forging heat treatment, and satisfactory structure and performance can still be obtained after the final heat treatment of the forgings. For example, coarse grains and Widmanstatten structure in general overheated structural steel forgings, slight network carbides caused by improper cooling of hypereutectoid steel and bearing steel, etc.
2. Some structural defects of forgings are difficult to eliminate with normal heat treatment, and can only be improved by measures such as high-temperature normalizing, repeated normalizing, low-temperature decomposition, and high-temperature diffusion annealing. 3. Some structural defects of forgings cannot be eliminated by general heat treatment processes, resulting in a decrease in the performance of the forgings after the final heat treatment, or even failure. For example, severe stone-like fracture and facet fracture, overburning, ferrite bands in stainless steel, carbide networks and bands in ledeburite high-alloy tool steel, etc.
4. Some structural defects of 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 the post-forging heat treatment, it often causes coarse martensite and unqualified performance after carbon and nitrogen co-diffusion and quenching; coarse banded carbides in high-speed steel often cause cracking during quenching. Different forming methods have different stress conditions and different stress-strain characteristics, so the main defects that may be produced are also different. For example, the main defects of billet upsetting are longitudinal or 45° cracks on the side surface, and only the upper and lower ends of the ingot upsetting often retain the cast structure; the main defects of rectangular cross-section billet elongation are transverse cracks and corner cracks on the surface, diagonal cracks and transverse cracks inside; the main defects of open die forging are insufficiency, folding and misalignment. Different types of materials have different compositions and structures, and their organizational changes and mechanical behaviors are different during heating, forging and cooling. Therefore, if the forging process is not appropriate, the defects that may occur are also particular. For example, the defects of ledeburite high alloy tool steel forgings are mainly coarse carbide particles, uneven distribution and cracks, and the defects of high temperature alloy forgings are mainly coarse grains and cracks; the defects of austenitic stainless steel forgings are mainly intergranular chromium depletion, reduced intergranular corrosion resistance, ferrite banded structure and cracks, etc.; the defects of aluminum alloy forgings are mainly coarse grains, folding, eddy currents, through-flow, etc.