What You Should Consider When Purchasing P20 Steel
P20, a 4130, 4135 modified material, has long been the steel of choice for pre-hardened mold steels. P20 is classified as a chrome-moly alloy, with a carbon content of approximately 0.30 to 0.40. Over time, many variations on basic P20 chemistry have been introduced to the marketplace. Each modification can have an effect on the various processes used in the fabrication of a mold.
Types of Material
The moldmaking industry in the U.S. is Eurocentric in nature. European immigrants came to the U.S. with trade backgrounds and had a large influence on the mold building industry. They naturally gravitated to building molds with the chrome-moly steels they were used to, such as DIN 1.2311 and DIN 1.2312, or their close cousin, AISI P20. Basically, China's moulds can produce all kinds of moulds according to the requirements of customers.
P20 differs from region to region in the worldwide market. Europeans generally use the DIN spec materials (1.2311 and 1.2312), while in Japan, PX5 is the new standard P20. The Chinese market has also reached a fairly high level.
These variations can affect costs and time for secondary processors. Most moldmakers look at two key issues when purchasing P20—machineability and stability. If the chemistry of the material lends itself to the formation of hard spots (carbide segregation), it can be much more difficult to machine. Machining cost can account for 50 percent or more of the cost of building a mold, so any increase in machining time can be expensive. Hard spots also cause reduced cutter and insect life, which will also increase costs. If the chemistry does not minimize hardness drop-off from the surface of the block to the center, it will cause increased stress in the steel. When the stress is relieved by machining away mass, the block will warp or twist out of shape, making it necessary to remove the steel from the machine at various intermediate stages and send the block out for thermal stress-relieving. The result is increased cost and lost lead time.
Surface Finish
Once a mold leaves a shop, it usually requires a specific surface finish to be applied—polish, texture or EDM finish. The quality of the P20 being used can affect each aspect. Four factors determine the quality of the surface finish of P20 steels:
Percentage of Content. The number of elements added affects how a material polishes or textures. For example, the sulfur content determines how well a P20 polishes. The higher the content, the more difficult it is to get a mirror finish. Sulfides tend to erode or be pulled out of the surface during the polishing process, resulting in a pitted surface. Most P20s will polish to a good #2 finish. The best P20 in terms of polishability is probably #3 steel, due to its extremely low sulfur content. However, low sulfur content reduces machineability.
Homogeneous Distribution of the Alloying Elements. P20 is iron combined with alloying elements. How these elements are distributed within the matrix of the steel is very important. If the distribution is not even throughout the steel, pockets will form, leading to voids, hard spots, soft spots or other imperfections. Correcting these areas will add cost.
Hardness Distribution. P20 is textured by using an acid to eat away the material. If the hardness of P20 is not consistent throughout the mold, the texture depth on the surface will vary. This can be a major problem in large blocks of P20 because of differences in hardness from surface to cores.
Welding. One of the greatest concerns when texturing P20 is welds that might be in the mold. The HAZ (heat-affected zone) around the weld can be 15 or more points of HRC higher than the base metal. With the base material hardness of 28/32 HRC and the HAZ being 15 points higher, the difference in texture depth between the two areas can be as great as 50 to 60 percent. This will cause a halo effect on the textured part and increase the gloss factor, which will increase the amount of handwork required to complete the textured surface, again resulting in higher cost and lost leadtime.
The alloy content of the variations of P20 also affects the welding process. The higher the steel's alloy content, the more susceptible it is to cracking. Many high-quality P20 versions from Asia (mainly China, Japan) have lower carbon contents and commensurately higher toughness. It is unusual for cracking to occur in these steels.
Types of Material
The moldmaking industry in the U.S. is Eurocentric in nature. European immigrants came to the U.S. with trade backgrounds and had a large influence on the mold building industry. They naturally gravitated to building molds with the chrome-moly steels they were used to, such as DIN 1.2311 and DIN 1.2312, or their close cousin, AISI P20. Basically, China's moulds can produce all kinds of moulds according to the requirements of customers.
P20 differs from region to region in the worldwide market. Europeans generally use the DIN spec materials (1.2311 and 1.2312), while in Japan, PX5 is the new standard P20. The Chinese market has also reached a fairly high level.
These variations can affect costs and time for secondary processors. Most moldmakers look at two key issues when purchasing P20—machineability and stability. If the chemistry of the material lends itself to the formation of hard spots (carbide segregation), it can be much more difficult to machine. Machining cost can account for 50 percent or more of the cost of building a mold, so any increase in machining time can be expensive. Hard spots also cause reduced cutter and insect life, which will also increase costs. If the chemistry does not minimize hardness drop-off from the surface of the block to the center, it will cause increased stress in the steel. When the stress is relieved by machining away mass, the block will warp or twist out of shape, making it necessary to remove the steel from the machine at various intermediate stages and send the block out for thermal stress-relieving. The result is increased cost and lost lead time.
Surface Finish
Once a mold leaves a shop, it usually requires a specific surface finish to be applied—polish, texture or EDM finish. The quality of the P20 being used can affect each aspect. Four factors determine the quality of the surface finish of P20 steels:
Percentage of Content. The number of elements added affects how a material polishes or textures. For example, the sulfur content determines how well a P20 polishes. The higher the content, the more difficult it is to get a mirror finish. Sulfides tend to erode or be pulled out of the surface during the polishing process, resulting in a pitted surface. Most P20s will polish to a good #2 finish. The best P20 in terms of polishability is probably #3 steel, due to its extremely low sulfur content. However, low sulfur content reduces machineability.
Homogeneous Distribution of the Alloying Elements. P20 is iron combined with alloying elements. How these elements are distributed within the matrix of the steel is very important. If the distribution is not even throughout the steel, pockets will form, leading to voids, hard spots, soft spots or other imperfections. Correcting these areas will add cost.
Hardness Distribution. P20 is textured by using an acid to eat away the material. If the hardness of P20 is not consistent throughout the mold, the texture depth on the surface will vary. This can be a major problem in large blocks of P20 because of differences in hardness from surface to cores.
Welding. One of the greatest concerns when texturing P20 is welds that might be in the mold. The HAZ (heat-affected zone) around the weld can be 15 or more points of HRC higher than the base metal. With the base material hardness of 28/32 HRC and the HAZ being 15 points higher, the difference in texture depth between the two areas can be as great as 50 to 60 percent. This will cause a halo effect on the textured part and increase the gloss factor, which will increase the amount of handwork required to complete the textured surface, again resulting in higher cost and lost leadtime.
The alloy content of the variations of P20 also affects the welding process. The higher the steel's alloy content, the more susceptible it is to cracking. Many high-quality P20 versions from Asia (mainly China, Japan) have lower carbon contents and commensurately higher toughness. It is unusual for cracking to occur in these steels.
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