[0001] The invention relates to a martensitic stainless steel article used for anchoring
molds and dies and to a method for producing the same.
DESCRIPTION OF THE PRIOR ART
[0002] Molds and dies used to produce parts made from materials such as plastic are anchored
in place during operation by frames, holder blocks, backers, and similar articles.
These articles are usually made from steel of a composition exhibiting high strength
and toughness to withstand the stresses incident to these applications and to provide
sufficient service life. The steel must also have good machinability to facilitate
manufacture of these articles and must be easily heat-treatable in relatively large
section sizes to the necessary hardness limits.
[0003] Typical steels used in the manufacture of frames and holder blocks are prehardened
within the hardness range of about 30 to 40 Rockwell C (HRC). This eliminates the
need for heat-treatment by the user, and avoids the distortion normally encountered
in heat-treating of machined articles. The hardness range of 30 to 40 HRC is significant,
because the machinability of most steels at hardnesses above 40 HRC is reduced to
a level that makes the required machining too expensive for most applications. Although
lowering the hardness of the steel improves machinability, at hardnesses below about
30 HRC the steel lacks sufficient mechanical strength for these intended applications.
[0004] The low-alloy carbon steels conventionally used for the production of holder blocks,
such as the sulfur-bearing modifications of AISI 4140 and AISI 5150, provide an excellent
combination of mechanical properties, in combination with good machinability. They,
however, lack sufficient corrosion resistance to resist rusting and other forms of
corrosion during both service and storage. This corrosive attack reduces the operating
safety, efficiency and service life and moreover requires that the holder blocks and
frames be covered with a protective coating when they are not in use.
[0005] A number of corrosion resistant steels have been evaluated as replacements for the
conventional low-alloy carbon steels used in holder block applications. High quality
stainless mold steels, such as AISI Type 414, AISI Type 420, and those disclosed in
U.S. Patent No. 3,720,545 have been considered; however, they are not widely used
for holder block applications because of their cost, properties, and comparatively
poor machinability. To overcome the machinability problem, a number of sulfur-bearing
modifications of AISI Type 420 and AISI Type 430 have been developed. While these
sulfur-bearing steels have relatively good machinability, they are not well suited
for this application because their inherent hardening and tempering characteristics
make it difficult to produce them in the broad hardness range of 30 to 40 HRC required
for holder blocks and especially in the narrower hardness range of 35 to 40 HRC required
for high strength holder blocks. In addition, the relatively high austenitizing temperatures
used to harden these steels, typically 1825 to 1900°F, result in increased cost and
contribute to considerable distortion of the articles during the hardening heat-treatment.
Further, at hardnesses within the range of about 30 to 40 HRC, these stainless steels
exhibit a characteristic drop in toughness and corrosion resistance that significantly
detracts from their usefulness in these applications.
OBJECTS OF THE INVENTION
[0006] It is a primary object of the present invention to provide a martensitic stainless
steel article which may be used for holder blocks, frames, backers, and similar articles
for anchoring molds and dies, having an improved combination of strength, toughness,
corrosion resistance, and machinability.
[0007] Another related object of the invention is to provide a method for producing a martensitic
stainless steel article having these characteristics by the use of a simple hardening
and tempering heat-treatment.
SUMMARY OF THE INVENTION
[0008] It has been determined in accordance with the invention that a martensitic stainless
steel article having an improved combination of strength, toughness, corrosion resistance,
and machinability may be produced by controlling carbon and nitrogen to achieve the
desired hardness. Sulfur is controlled in accordance with carbon plus nitrogen to
maintain a drill machinability rating equal to or greater than 100 (when compared
to a commercial stainless holder block steel). For this purpose, sulfur must be increased
with increases in the carbon and nitrogen content. Chromium, and also nickel, are
present for maintaining corrosion resistance. Molybdenum is also added for corrosion
resistance and specifically to counteract the adverse effects of increased sulfur
in this regard. Consequently, molybdenum is increased with increased sulfur contents.
[0009] In accordance with the invention, a martensitic stainless steel article, which may
be used for holder blocks, frames, backers, and similar articles for anchoring molds
and dies, is of a composition within the limits set forth in Table 1.

[0010] The article is characterized by a hardness within the range of 30 to 40 HRC, preferably
35 to 40 HRC for higher strength applications.
[0011] In accordance with the method of the invention, steel in accordance with the composition
limits set forth in Table I is austenitized at a temperature within the range of 1500
to 1750°F for about 1 hour per inch of thickness and either oil quenched or air cooled
to achieve a martensitic structure. Thereafter, the article can be tempered or stress-relieved
at a temperature between about 500 and 850°F for about 1 hour per inch of thickness
and for a minimum of 2 hours. After these heat treatments, the articles will exhibit
a hardness within the range of 30 to 40 HRC, preferably 35 to 40 HRC for high strength
applications, and a drill machinability rating equal to or greater than 100.
[0012] Preferably, the article after tempering in addition exhibits a corrosion rate in
inches per year of less than 9 when tested in accordance with the procedure disclosed
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013]
Figure 1 is a graph showing the relationship between tempering temperature and hardness
for a commercial stainless holder block steel of the composition, in weight percent,
0.32% carbon, 1.33% manganese, 0.32% silicon, 0.097% sulfur, 0.50% nickel, 16.8% chromium,
0.04% molybdenum, 0.034% nitrogen and balance iron and incidental impurities;
Figure 2 is a graph showing the relationship between tempering temperature and hardness
for the two indicated holder block steels in accordance with the invention;
Figure 3 is a graph showing the relationship between the hardness of holder block
steels in accordance with the invention in the as-hardened condition in relation to
the carbon plus nitrogen content thereof;
Figure 4 is a graph showing the relationship between the drill machinability of holder
block steels in accordance with the invention with respect to a parameter relating
to the hardness and sulfur contents thereof; and
Figure 5 is a series of photographs comparing the corrosion resistance of three holder
block steels in accordance with the invention with two holder block steels of compositions
outside the scope of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Stainless steel holder blocks are generally made by hot rolling or forging an ingot
to slab or billet that is subsequently heat-treated to the desired final hardness
and then sawed and machined into blocks of the required shapes and dimensions. Less
commonly, the holder blocks are cut and rough machined from fully annealed slabs or
billets, heat-treated separately to the desired hardness, and then machined to final
shape. The hardness typical of standard holder block applications ranges from about
30 to 35 HRC, whereas that for high strength holder block applications ranges from
about 35 to 40 HRC. In order to attain these hardnesses without undue cost or difficulty,
it is essential that the steel used in the holder block be readily heat-treatable
to the required hardness levels. With stainless steels typical of those now used in
corrosion resistant holder blocks, such as that tested to obtain the data presented
in Figure 1, the tempering temperatures required to produce hardnesses in the range
of about 30 to 40 HRC and especially in the range of about 35 to 40 HRC are quite
critical in that slight differences in temperature result in a large difference in
hardness. Thus, very close control of the tempering operation is needed with these
steels to obtain the hardnesses required for holder block applications. Further, such
steels when tempered to hardnesses in the range of about 30 to 40 HRC exhibit relatively
low notch toughness and corrosion resistance.
[0015] In comparison, with the holder block steels of this invention it is possible to obtain
an improved combination of corrosion resistance and toughness and the hardnesses needed
for this application with a simple heat-treatment. Figure 2 shows that steel holder
blocks produced in accordance with the invention and within the composition limits
given in Table I provide the desired hardnesses in both the as-hardened condition
and when tempered or stress-relieved over a broad range of temperatures. For example,
a steel holder block made from Heat V1056 containing 0.043% carbon plus nitrogen achieves
a hardness well within the range needed for standard holder blocks (30 to 35 HRC)
in the as-hardened condition and also when tempered or stress relieved at temperatures
up to about 850°F. Similarly, a holder block made from Heat V1020 with 0.079% carbon
plus nitrogen achieves a hardness well within the range 35 to 40 HRC needed for high
strength holder blocks in the as-hardened condition and also when tempered or stress
relieved over a wide range of temperatures. Also, in contrast to stainless steels
of the type now used in corrosion resistant holder blocks, which are normally austenitized
from temperatures between about 1825 to 1900°F, steel holder blocks produced within
the scope of the invention can be austenitized from temperatures as low as about 1550°F,
which achieves considerable energy savings in heat-treatment.
[0016] With respect to the chemical composition of the steels used in the holder blocks
of this invention, it is necessary within the composition ranges given in Table I
to control their overall composition so that the holder blocks will be substantially
fully martensitic in the as-hardened condition. To obtain a substantially fully martensitic
structure in the as-hardened condition, it is necessary that the composition of the
steels be balanced with respect to the austenite forming elements, such as carbon,
nitrogen, nickel, and manganese, and the ferrite forming elements, such as chromium,
molybdenum, and silicon, to minimize the formation of delta ferrite. Large amounts
of delta ferrite are detrimental in the steel from the standpoint of reducing the
hardness and toughness of holder blocks made therefrom.
[0017] The hardness of the steels used in the holder blocks of the invention in the as-hardened
condition is primarily a function of the carbon plus nitrogen content. To obtain the
desired hardnesses within the range of 30 to 40 HRC, it is therefore necessary to
control the carbon and nitrogen contents within the ranges indicated in Table I. With
a carbon plus nitrogen content that is too low, the holder blocks will not achieve
the minimum desired strength and hardness; with a carbon plus nitrogen content that
is too high, the holder blocks will exceed the desired maximum hardness and exhibit
unacceptable machinability.
[0018] Manganese is a desirable element in the steels used in the holder blocks. Manganese
imparts hardenability and, in combination with sulfur, is also present for purposes
of improving machinability through the formation of manganese sulfide. Also, manganese
is an austenite forming element and can be used to partially replace nickel in the
steel for composition balance and to thereby reduce steel costs.
[0019] Silicon is used in steelmaking for deoxidation and increasing chromium recovery.
It also slightly improves corrosion resistance, but is a ferrite forming element and
thus increases the amount of costly nickel or manganese needed to obtain a fully martensitic
structure.
[0020] Nickel is required within the indicated ranges to obtain the desired austenite-ferrite
balance and to thereby obtain a substantially fully martensitic structure in the holder
blocks. It also improves corrosion resistance; but is a costly element, and for this
reason is not desirable above the indicated ranges.
[0021] Chromium is essential for corrosion resistance, but above the indicated amounts increases
the amount of nickel, manganese, and other austenite forming elements that are required
to be present to avoid the formation of delta ferrite and to obtain a substantially
fully martensitic structure in the holder blocks.
[0022] Molybdenum is an expensive alloying element, but in small amounts and together with
chromium has a very beneficial affect on the corrosion resistance of the holder blocks,
and a minimum of about 0.25% is necessary for reducing the adverse effects of sulfur
on this property. Consequently, molybdenum generally should be increased in the presence
of increased sulfur for this purpose.
[0023] Sulfur is used for improving machinability, but decreases notch toughness and corrosion
resistance. When high toughness and corrosion resistance are required in the holder
blocks of the invention, sulfur should be limited to about 0.10%; but when greater
machinability is desired, it can be increased to about 0.25% without lowering toughness
and corrosion resistance to unacceptable levels. Molybdenum should be increased with
increased sulfur to maintain corrosion resistance at the desired level.
[0024] Copper is a common residual element in stainless steel melting, and is useful for
controlling the austenite-ferrite balance. However, in amounts greater than about
1.0% it can have an undesirable hardening effect during tempering of the holder blocks.
[0025] To demonstrate the principles of the invention, a series of experimental holder block
steels were made and subjected to a variety of mechanical and corrosion tests. The
chemical compositions of the experimental holder block steels and of a commercial
stainless holder block steel (Alloy 90-45) included for comparison are given in Table
II.

[0026] Ingots of the experimental holder block steels were hot worked from a reheating temperature
of about 2150°F to bar stock from which samples were taken for metallographic evaluation
and testing. Except for those samples used to determine attainable hardness, all the
test samples were austenitized at 1550°F, air cooled to room temperature, and then
tempered for two hours at 550°F. None of the experimental holder block steels were
found to contain any delta ferrite after this heat-treatment. The samples of the commercial
stainless holder block steel were received in the prehardened condition at a hardness
of 33 HRC. In order to test this material at a higher hardness of 38 HRC, samples
of the commercial holder block steel were austenitized at 1850°F, oil quenched to
room temperature and then tempered for 2 hours at 975°F.
[0027] Several tests were conducted to compare the advantages of the holder block steels
of the invention with those of a commercial stainless holder block steel and to demonstrate
the significance of their composition. Tests were conducted to illustrate the effects
of steel composition on attainable hardness, notch toughness, tensile strength, machinability,
and corrosion resistance.
[0028] The attainable hardnesses of the experimental holder block steels in the as-hardened
condition are plotted in Figure 3 as a function of their carbon plus nitrogen contents.
The specimens for these tests were austenitized for 15 minutes at 1600°F and then
air cooled to room temperature. Allowing for some normal scatter in the results of
the hardness tests, Figure 3 shows that the attainable hardness of the steels used
in the holder blocks of the invention has a strong relationship with their carbon
plus nitrogen contents. To obtain the hardnesses needed for holder block applications
(30 to 40 HRC), Figure 3 shows that the carbon plus nitrogen contents of the holder
blocks of the invention must be controlled in a range between about 0.02 to 0.09%.
Further, to obtain the hardness typical of standard holder blocks (30 to 35 HRC) and
of high strength holder blocks (35 to 40 HRC), the carbon plus nitrogen content of
the steels used in the holder blocks of the invention must be controlled from about
0.02 to 0.06% and from about 0.06 to 0.09%, respectively.
[0029] The results of the notch toughness and tension tests conducted on the experimental
holder block steels and on the commercial stainless holder block steel are given in
Table III.

These test results show that the notch impact toughness of the steels used in the
holder blocks of this invention, as measured in the Charpy V-notch impact test, are
clearly superior to those of a commercial stainless steel typically used in this application
(Alloy 90-45). The advantage in toughness is particularly great for those experimental
steels containing less than about 0.10% sulfur, as can be seen by comparing the notch
toughness values of Alloy V1033 (30.6 ft-lb) with those of the commercial stainless
holder block steel (5.0 ft-lb). Above sulfur levels of about 0.10%, the impact properties
of the steels used in the holder blocks of the invention are still significantly better
than that of the commercial stainless holder block steel. For example, the notch toughness
of Alloy V1055 with 0.20% sulfur is 15.0 ft-lb in the longitudinal direction; whereas,
that of the commercial stainless holder block steel (Alloy 90-45) with 0.09% sulfur
is only 5.0 ft-lb.
[0030] The tensile properties of the steels used in the holder blocks of this invention
are largely a function of their hardness and are at least comparable to those of the
commercial stainless holder block steel at the same hardness. About the same mechanical
properties and notch toughness are obtained for the higher manganese and lower nickel
containing experimental holder block steels (Alloys V1022 and V1055) as for the comparable
steels with higher nickel and lower manganese (Alloys V1020 and V1056). Thus, when
it is desirable to reduce cost, manganese can be used to replace part of the nickel
in the steels used in the holder blocks of this invention.
[0031] The results of drill machinability tests conducted on the experimental steels used
in the holder blocks of the invention and on a commercial stainless holder block steel
are given in Table IV and in Figure 4. The machinability indexes given in this table
and figure were obtained by comparing the times required to drill holes of the same
size and depth in the experimental steels and in the commercial stainless holder block
steel at a hardness of 33.0 HRC and by multiplying the ratios of these times by 100.
Indexes greater than 100 indicate that the drill machinability of the test specimen
is greater than that of the commercial stainless holder block steel. Because the hardness
and sulfur content of these steels are known to influence machinability, a parameter
based on these factors [Rockwell C hardness -100 (% S)] was derived and used to compare
the drill machinability of the test materials.

[0032] Analysis of the drill machinability test data using the relationship derived between
the above parameter and the machinability index indicates that to provide machinability
at least equivalent to that of the commercial stainless holder block steel at a hardness
of 33 HRC, the steels used in the holder blocks of this invention must contain at
least 0.05% sulfur. Likewise, to provide machinability at least comparable to that
of the commercial stainless holder block steel at a hardness of 33 HRC, the holder
block steels of the invention at a hardness of 38 HRC must contain at least 0.10%
sulfur. These results, in combination with those of the notch toughness tests reported
in Table III, indicate that at sulfur levels between about 0.05 and 0.10% the steels
used in the holder blocks of the invention afford substantially better notch toughness
and machinability superior to that provided by current stainless holder block steels.
They also indicate that at sulfur contents between about 0.10 and 0.25%, the steels
used in the holder blocks of this invention provide substantially better machinability
and notch toughness superior to that of current stainless holder block steels.
[0033] Two tests were used to compare the corrosion resistance of the steels used in the
holder blocks of this invention to that of a typical commercial stainless holder block
steel, the composition of which is given in Table II. In one test, the weight loss
and resulting corrosion rates were determined for specimens immersed for three hours
at ambient temperature in a dilute solution of aqua-regia containing 5% nitric acid
and 1% hydrochloric acid by volume. This test is described in the literature (E. A.
Oldfield, "Corrosion of Cutlery", Corrosion Technology, June, 1958, pp. 187-189) and
is particularly useful for comparing the effects of composition and heat treatment
on the corrosion resistance of martensitic stainless steels. The term "corrosion rate
in inches per year" as used herein refers to the corrosion rate exhibited by an alloy
article subjected to this test procedure. These tests were conducted on specimens
that were passivated and not passivated prior to testing in a solution of 20% nitric
acid containing 3% by weight of potassium chromate at 120°F for 1/2 hour. The other
test was a salt spray test in which specimens were exposed for three hours at 90°F
to vapors generated from an aqueous solution containing 2.5% by weight of sodium chloride.
In this latter test, material performance was ranked visually by estimating the percentage
of the surface area that was affected by corrosion. The results of the corrosion tests
are summarized in Table V. Photographs of five of the specimens subjected to the salt
spray test are shown in Figure 5.

[0034] The results of the dilute aqua-regia and the salt spray tests clearly show that the
steels used in holder blocks of this invention have substantially better corrosion
resistance than a steel typical of that now used in stainless steel holder blocks.
This is evidenced by the great difference in the corrosion rates exhibited in the
dilute aqua-regia test by Alloys V1033 (4.3 inches/year) and V1021 (5.5 inches/year),
whose compositions are within the scope of the invention, and Alloy 90-45 (14.1 inches/year)
which is representative of the steels now used in stainless steel holder blocks. The
great advantage of the steels used in the holder blocks of this invention is also
exhibited in the salt spray test, as can be seen by comparing the percent affected
area for these same alloys. The results of the corrosion tests also demonstrate the
importance of maintaining the molybdenum content of the steels used in the holder
blocks of this invention above about 0.25. In this regard, note, for example, the
relatively poor performance of Alloy V1087, which except for a very low molybdenum
content had a composition within the scope of the invention, as compared to the good
performance of Alloys V1003 and V1009, which contain about 0.32% molybdenum and whose
compositions are within the scope of the invention.
[0035] The relative corrosion resistance of three of the experimental holder block steels
(Alloys V1009, V1020, and V1020) and of two steels (Alloys V1087 and 90-45) outside
the scope of the invention is further illustrated in Figure 5. As can be seen, Alloys
V1009, V1020, and V1021, having compositions within the scope of the invention, show
considerably better corrosion resistance in the salt spray test than do Alloys V1087
and 90-45. The composition of Alloy V1087 is similar to that of Alloys V1009 and V1020,
except that it contains less than 0.01% molybdenum. This again demonstrates the importance
of maintaining a minimum of about 0.25% molybdenum in the steels used in the holder
blocks of this invention. Alloy 90-45 is typical of the steels currently used in stainless
steel holder blocks, and its comparatively poor performance again demonstrates that
the steels used in the holder blocks of this invention have substantially better corrosion
resistance.
[0036] The results of the corrosion tests together with those of the mechanical property
tests in Table III and of the machinability tests in Table IV clearly show that the
corrosion resistant holder block steels of the invention provide a substantially better
combination of notch toughness, machinability, and corrosion resistance than afforded
by conventional stainless steel holder blocks. Further, the steels used in the holder
blocks of the invention have the advantage of being hardenable to the hardnesses needed
for this application with a simple heat-treatment.
1. A martensitic stainless steel article, which may be used for holder blocks, frames,
backers, and similar articles for anchoring molds and dies, said article being characterised
by having a hardness within the range of 30 to 40 HRC and comprising, in weight percent,
up to 0.09% carbon, up to 0.09% nitrogen, 0.02 to 0.09% carbon plus nitrogen, up to
4.50% manganese, up to 0.05% phosphorus, 0.05 to 0.25% sulfur, up to 1.0% silicon,
1.00 to 4.00% nickel, 11.00 to 14.00% chromium, 0.25 to 1.00% molybdenum, up to 1.00%
copper, balance iron and incidental impurities.
2. A martensitic stainless steel article according to claim 1 wherein the hardness is
30 to 35 HRC, and in weight percent, the amount of carbon is up to 0.06% the amount
of nitrogen is up to 0.06% the amount of carbon plus nitrogen is 0.02% to 0.06%.
3. A martensitic stainless steel article according to claim 1 wherein the hardness is
35 to 40 HRC and the amount, in weight percent of carbon plus nitrogen is 0.06% to
0.09%.
4. A martensitic stainless steel article according to any of claims 1 to 3 wherein in
weight percent, the amount of chromium is 11.00 to 13.00% and the amount of molybdenum
is 0.25 to 0.75%.
5. A martensitic stainless steel article according to any one of the preceding claims
wherein in weight percent, the amount of manganese is up to 2.00% and the amount of
nickel is 2.00 to 4.00%.
6. A martensitic stainless steel article according to any one of claims 1 to 4 wherein
in weight percent, the amount of manganese is 2.00 to 4.50% and the amount of nickel
is 1.00 to 2.00%.
7. A martensitic stainless steel article according to any one of the preceding claims
wherein, in weight percent, the amount of sulphur is 0.05 to 0.10%.
8. A martensitic stainless steel article according to any one of claims 1 to 6 wherein,
in weight percent, the amount of sulphur is 0.10 to 0.25%.
9. A method for producing a martensitic stainless steel article, which may be used for
holder blocks, frames, backers, and similar articles for anchoring molds and dies,
said method being characterised by comprising producing said article of an alloy composition
consisting essentially of, in weight percent, up to 0.09% carbon, up to 0.09% nitrogen,
0.02 to 0.09% carbon plus nitrogen, up to 4.50% manganese, up to 0.05% phosphorus,
0.05 to 0.25% sulfur, up to 1.0% silicon, 1.00 to 4.00% nickel, 11.00 to 14.00% chromium,
0.25 to 1.00% molybdenum, up to 1.00% copper, balance iron and incidental impurities;
austenitizing said article at a temperature of 1500 to 1750°F (833 to 972°C) for about
1 hour per inch of thickness and thereafter air cooling or oil quenching to achieve
a martensitic structure and thereafter tempering or stress-relieving said article
at a temperature of 500 to 850°F (278 to 472°C) for about 1 hour per inch (2.54 cm)
of thickness and for a minimum of 2 hours to achieve a combination of a hardness within
the range of 30 to 40 HRC and a drill machinability rating equal to or greater than
100.
10. A method according to claim 9 wherein the amount of carbon plus nitrogen is 0.06 to
0.09 weight percent.
11. A method according to claim 9 wherein, in weight percent, the amount of carbon is
up to 0.06%, the amount of nitrogen is up to 0.06% and the amount of carbon plus nitrogen
is 0.02 to 0.06%.
12. A method according to any one of claims 9 to 11 wherein, in weight percent, the amount
of chromium is 11.00 to 13.00% and the amount of molybdenum is 0.25 to 0.75%.
13. A method according to any one of claims 9 to 12 wherein, in weight percent, the amount
of manganese is 2.00 to 4.50% and the amount of nickel is 1.00 to 2.00%.
14. A method according to any one of claims 9 to 12 wherein, in weight percent, the amount
of manganese is up to 2.00% and the amount of nickel is 2.00 to 4.00%.
15. The method of any one of claims 9 to 14 wherein said alloy composition has sulfur
of 0.05 to 0.10%.
16. The method of any one of claims 9 to 14 wherein said alloy composition has sulfur
of 0.10 to 0.25%.
17. The method of any one of claims 9 to 16 wherein said article after said tempering
exhibits a corrosion rate in inches per year of less than 9 (22.86 cm).