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Moldmaking
Technology Magazine
What You Should Consider When Purchasing P20
Steel
When buying P20 steels, moldmakers should be aware of the
variations and key issues that affect delivery, cost and
leadtimes.
by Paul W. Britton
November, 2004
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 .30 to .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.
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 is less clear, because the lack of information
about the materials being used for their molds makes it difficult
for secondary polishing and texturing processors to know what they
are dealing with. The U.S. market is a melting pot of different
grades; Figure 1 shows a chemical breakdown of the materials
used in the U.S. As the figure illustrates, there are variances
within the P20 family, and depending on the steel supplier, an order
for P20 could be filled with nearly any one of them. Two of the
items listed in the table (P20 premium and high hard P20) are
actually closer in chemistry to a 4340 material, which is a
chrome-nickel-moly material. As noted earlier, P20 is in the 4130,
4135, 4140 (chrome-moly) family.
| Figure 1: P20
Materials Analysis |
| GRADE |
C |
Si |
Mn |
S |
Cr |
Mo |
Ni |
V |
AISI P20
|
0.350
|
0.450
|
0.850
|
0.035
|
1.800
|
0.500
|
|
|
P20 MODIFIED
|
0.400
|
0.300
|
1.400
|
0.003
|
2.000
|
0.200
|
|
|
P20 MOLD QUALITY
|
0.310
|
0.400
|
0.750
|
0.008
|
1.200
|
0.410
|
|
|
P20 PREMIUM
|
0.400
|
0.300
|
1.400
|
0.003
|
2.000
|
0.200
|
1.000
|
|
DIN 1.2311
|
0.400
|
0.300
|
1.450
|
0.035
|
1.950
|
0.200
|
|
|
DIN 1.2312
|
0.400
|
0.400
|
1.500
|
0.080
|
1.900
|
0.200
|
|
|
JIS PX5
|
0.200
|
|
1.250
|
0.050
|
2.000
|
0.400
|
|
0.100
|
HIGH HARD P20
|
0.400
|
0.300
|
1.500
|
0.010
|
2.000
|
0.200
|
1.000
|
|
#3 STEEL
|
0.300
|
0.250
|
0.500
|
|
0.950
|
0.200
|
|
|
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 insert 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 leadtime.
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 steels1:
1. 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.
2. 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.
3. 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.
4. 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 Japan) have lower carbon
contents and commensurately higher toughness. It is unusual for
cracking to occur in these steels. The lower carbon content also
keeps the hardness rise of the HAZ to just a few points. As the
carbon content and the content of other alloys (such as nickel) are
increased through the standard P20s and some of the high hard P20s -
as well as on some of the DIN spec materials - pre- and post-heating
becomes critical to controlling cracking and extreme hardness rise
in the HAZ. This can add time and cost to the welding process.2
Making A Decision
Extreme pressure is being applied to leading steel producers in the
U.S., Europe and Japan to develop new, advanced grades of mold
steels. New melt and refining technologies, alloys and free
machining additives are, or will be, available to moldmakers. The
new steels these producers develop, properly used, will allow
moldmakers to be the best and most competitive in the world.
Until then, U.S. moldmakers will
continue to use P20 because they understand the basic problems
associated with the grade, and although it is costly, they can
overcome those problems. It is like fighting with an old friend.
However, moldmakers must keep in mind that overcoming most P20
problems requires that extra time and cost be built into the job. In
the cost-reducing, leadtime-reducing world ecomony, to be
competitive moldmakers must eliminate these problems by looking at
key issues that affect delivery and cost when purchasing variants of
P20.
For more information contact Paul
Britton of International Mold Steel, Inc. (Florence, KY) at (800)
625-6653 or visit the Web site at www.imsteel.com.
References
1Larry Taylor of CST
(Complete Surface Technology) (Clinton Township, MI).
2Derek
Starks, vice president of Accurate Welding (Detroit, MI).
Reprinted with
permission from Communications Technologies, Inc
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International Mold Steel, Inc.