Selecting Stainless Steels For Valves
February 2005
By Ralph Vedder, Staff Specialist, Bar Technical Services
Carpenter Technology Corp., Reading, PA, USA
Technological advances in valve design and use over the past decade present
interesting challenges today for those engaged in valve manufacturing and
applications. Among these challenges is how to select the "best" material for
components and a wide variety of end-use products.
More particularly, which of more than 400 commercially available stainless
steels is the "right" one for a specific application, with newly introduced
alloys constantly confusing the issue? The alloys pertinent to this discussion
have been used typically in the industrial, chemical, petrochemical,
pharmaceutical and semi-conductor industries.
While the selection process must focus on the steel’s properties and
characteristics, the designer/engineer making the choice also knows that
life-cycle costs, or the total cost over the equipment’s or product’s design
life, is of bottom-line importance. The slightly higher cost of a more
corrosion-resistant stainless steel, for example, may dwarf the total cost of
settling for a next-best alloy. That’s because an unwise choice can be burdened
with the additional cost of maintenance and replacement, plus the cost of lost
production during equipment downtime.
Some newer valve applications, like the most stringent ones in the
pharmaceutical and semi-conductor industries, limit material choice. For those
critical applications requiring highly polished internal surfaces, the stainless
steel selected must be premium melted to assure a super clean microstructure.
Otherwise, parts that must be electro polished will be subject to pitting and
failure that could be catastrophic in a production process.
Selection Criteria
Given this realistic value analysis, selection of the most suitable alloy should
be based largely on five criteria. They are:
1. Corrosion Resistance. Be sure to consider the environment and type of
corrosion resistance to be encountered. The most cost effective stainless steel
will have enough corrosion resistance to provide the required service life
without the needless expense of over-alloying.
2. Mechanical Properties. Strength is usually the most important
mechanical property in the selection of a stainless steel. However, the designer
may be concerned also with the alloy’s hardness, ductility, impact resistance,
fatigue strength or stress rupture resistance – or, perhaps more than one of
these properties. Furthermore, the potential effect of the desired mechanical
property on the alloy’s corrosion resistance may be important.
3. Physical Properties. These could include density or magnetic
properties. For example,
22Cr-13Ni-5Mn (UNS S20910) stainless steel, which is non-magnetic in
all conditions, may be preferred to
Type 304 stainless steel, which may exhibit ferro-magnetic properties
when cold worked.
4. Fabrication. Will the component be machined, forged, cold formed or
welded? The fabrication required should be considered from three perspectives:
(a) How easy or difficult will it be to fabricate the component? (b) How will
fabrication affect the alloy’s performance? (c) Will the quality of the alloy
permit satisfactory fabrication?
5. Final Component Cost. Do a thorough value analysis that
includes the initial alloy price, the installed cost, and the effective life
expectancy of the finished product.
Corrosion Resistance
The Selectaloy® method, developed by Carpenter, is an effective means of
selecting stainless steels. It begins with a determination of how much corrosion
resistance is needed, and then factors in strength requirements for the specific
application.
When looking for the right stainless steel, it is best to start with
Type 304/304L stainless, the alloy specified more than 50% of the
time whenever a stainless steel is used. It resists most oxidizing acids and
general corrosion in many environments.
For greater corrosion resistance, move up to
Type 316/316L stainless. This stainless is like Type 304, but with
added molybdenum to boost its resistance to the attack of many industrial
chemicals and solvents, and especially pitting caused by chlorides. A quality
upgrade of Type 316 stainless known as
Carpenter Type 316L-SCQ® stainless has been found useful for valving
in the semi-conductor industry where the components can be exposed to halide
gases.
For more severe corrosion environments,
20Cb-3® stainless should be considered. This is a highly alloyed
stainless steel that offers special resistance to sulfuric acid, and protection
against loss of corrosion resistance from welding. It offers superior resistance
to hundreds of industrial and process corrodents, especially in valved fluid
piping systems.
There are many applications that require less corrosion resistance than that
exhibited by the three grades mentioned. In those instances,
Type 430 stainless, which is a less costly stainless alloy, may
provide an adequate level of corrosion resistance. While it has less corrosion
resistance than Type 304 stainless, it does resist foodstuffs, fresh water and
non-marine atmospheric atmosphere.
For those applications requiring only the minimum resistance to corrosion, Type
405 stainless may be specified. It resists corrosion from soap, alcohol, crude
oil, gasoline, mercury and other mild reagents.
The five stainless steels discussed so far are plotted in Figure 1 in order of
increasing corrosion resistance. The basic Type 304 stainless is positioned in
the middle.
Fig. 1 – Stainless steels in order of increasing corrosion resistance.
Mechanical Properties
After the appropriate level of corrosion resistance has been ascertained, a
careful study of mechanical properties is necessary to determine the proper
alloy for an application. If low-level corrosion resistance to corrosion is
adequate, but higher strength is required,
Type 410 stainless should provide the service desired. Type 410
offers a wide range of mechanical properties, depending upon the tempering
temperatures employed. It has been useful in applications demanding good
strength, ductility and resistance to oxidation up to 649°C (1200°F).
For greater strength and hardness at the same level of corrosion resistance,
Type 420 stainless may be considered. It has higher strength and wear
resistance in the hardened and tempered condition than Type 410 stainless.
Type 440C stainless combines the lowest level of corrosion resistance with very
high hardness and strength. This alloy may be considered for a broad range of
products that require high hardness values and wear resistance.
combines the lowest level of corrosion resistance with very high
hardness and strength. This alloy may be considered for a broad range of
products that require high hardness values and wear resistance.
The selection diagram has now been expanded to include, in Figure 2, the
stainless grades with the lowest level of corrosion resistance, but with
increasing strength moving left to right. In many valve applications, fluids
must be regulated at different pressure levels. That is an application design
factor that should be considered in finding a stainless with the right strength
for the job.
Fig. 2 - Stainless steels in order of increasing strength.
For Greater Strength
If greater strength is required, at a good level of corrosion resistance, move
right on the chart directly opposite Type 430 to the thermally hardenable
Type 431 stainless. With its good combination of corrosion
resistance, toughness and hardness, this alloy may be considered for temperature
applications between 73°C (-100°F) and 649°C (1200°F).
For highest strength at the second level of corrosion resistance,
Custom 455® stainless (UNS S45500) is designated. This unique alloy
can be used wherever a combination of high strength, good corrosion resistance,
simple heat treatment and ease of fabrication is required.
Custom 450® stainless (UNS S45000) offers an unusual combination of formability
and high strength, along with corrosion resistance that is unique for a material
with 689 MPa (100 ksi) minimum yield strength. Its yield strength can be
increased 50% through a simple, one-step, low temperature aging process without
significantly decreasing the alloy’s corrosion resistance. The complete
Selectaloy diagram is shown in Figure 3.
(UNS S45000) offers an unusual combination of formability and high
strength, along with corrosion resistance that is unique for a material with 689
MPa (100 ksi) minimum yield strength. Its yield strength can be increased 50%
through a simple, one-step, low temperature aging process without significantly
decreasing the alloy’s corrosion resistance. The complete Selectaloy diagram is
shown in Figure 3.
For greater strength requirements, a newer alloy known as
Custom 465® stainless (UNS S46500), can be considered. This
stainless, which would be positioned to the right of Custom 450 stainless in the
diagram, is a premium melted, martensitic, age-hardenable alloy capable of
ultimate tensile strength in excess of 250 ksi (1724 MPa) in the overaged (H950)
condition. This alloy was designed to have excellent notch tensile strength and
fracture toughness in this condition. Overaging to the H1000 condition provides
a superior combination of strength, toughness and stress corrosion cracking
resistance compared with other high-strength PH stainless alloys.
Fig. 3 – Selectaloy® diagram showing relative corrosion resistance and strength
of 11 basic stainless steels.
Nitrogen Strengthened Alloys
Many applications require a balanced combination of improved strength and
corrosion resistance. When seeking greater strength with good corrosion
resistance, the specifier should check the family of nitrogen-strengthened
alloys shown in the modified Selectaloy diagram (Figure 4). All four have
comparable mechanical properties, with yield strength of 345 MPa (50 ksi) to 414
MPa (60 ksi) as annealed, and strength levels in excess of 689 MPa (100 ksi)
when cold worked.
These alloys are austenitic stainless steels with nitrogen added for improved
strength and corrosion resistance. All of them, except
Gall-Tough® stainless (UNS S20161), remain nonmagnetic even after
severe cold working.
The group starts with the
18Cr-2Ni-12Mn stainless (UNS S24100), which has corrosion resistance
similar to Type 430 stainless. It offers an excellent combination of toughness,
ductility, corrosion resistance, strength and good fabricability. Farther up the
scale are Gall-Tough stainless and
21Cr-6Ni-9Mn stainless (UNS S21904). These two grades can be
considered for valve applications because they have corrosion resistance similar
to that of Type 304 stainless with twice the yield strength and excellent high
temperature strength.
Gall-Tough PLUS stainless (UNS S20162), a variation of Gall-Tough stainless, is
a high-silicon, high-manganese, nitrogen-strengthened, austenitic stainless
alloy that exhibits superior self-mated galling and metal-to-metal wear
resistance. The alloy exhibits higher strength than Type 316. It also exhibits
chloride corrosion resistance equal to or better than Type 316 stainless, along
with equivalent high temperature oxidation resistance.
(UNS S20162), a variation of Gall-Tough stainless, is a high-silicon,
high-manganese, nitrogen-strengthened, austenitic stainless alloy that exhibits
superior self-mated galling and metal-to-metal wear resistance. The alloy
exhibits higher strength than Type 316. It also exhibits chloride corrosion
resistance equal to or better than Type 316 stainless, along with equivalent
high temperature oxidation resistance.
Fig. 4 – Modified Selectaloy® diagram with the addition of four
nitrogen-strengthened stainless steels.
Gall-Tough stainless can be considered for applications such as valve, pump
components and shafts where surface-to-surface friction is an issue. The
21Cr-6Ni-9Mn stainless may be considered also for valve applications requiring
good corrosion resistance with superior strength characteristics.
The most corrosion resistant stainless in this family is the
22Cr-13Ni-5Mn grade. This alloy has better corrosion resistance than
Type 316 stainless, and twice the yield strength. It provides high-level
resistance to pitting and crevice corrosion and very good resistance in many
reducing and oxidizing acids and chlorides.
Fabrication
After the stainless steel is selected on the basis of corrosion resistance and
mechanical properties, it’s time to consider how the part or component is to be
fabricated. The specifier may want to improve fabrication characteristics by
using a modification of the alloy chosen. There are many modifications of the 15
basic stainless steels mentioned, with individual and distinctive fabrication
properties. Each has a different name or type number depending on its chemical
analysis, and each constitutes an addition to the growing family of stainless
steels.
Suppose, for example, we have chosen the basic Type 304 stainless for an
application, and anticipate a difficult machining problem. Although Type 304
stainless can be machined, other variations of the alloy have better
machinability.
Two other members of the group offer better machinability. In order of improving
machinability, they are
Project 70+® Type 304 stainless and
Project 70+ Type 303 stainless. In addition,
Project 70+ Type 316 stainless offers the superior corrosion
resistance of its conventional counterpart, along with improved machinability.
As the designer or engineer moves about within the fabrication families,
searching for a suitable alloy modification, it is important to note that
reaching the ultimate material for one fabrication process is achieved at some
loss in other fabrication qualities. For example, as machinability improves, the
more likely the alloy’s cold working characteristics will decline.
Summary
Several key factors should be considered in selecting the most suitable
stainless steel for any use. First, with the diagram provided, select the level
of corrosion resistance required. Second, choose the level of strength needed.
Third, consider the type and amount of fabrication necessary; then select the
alloy modification which offers the most desirable fabricating characteristics.
Do a thorough value analysis, which includes the initial alloy price, the
installed cost and the effective life expectancy of the finished product.
Finally, determine the availability of the preferred alloy from the steel mill,
service center, warehouse or supplier to arrive at the most economical and
practical choice.
