By John
Magee and Tinsley Fehnel
Carpenter Technology Corp., Wyomissing, PA, USA
November 2006
Hardenable
stainless steel alloys are a common choice for manufacturers that cold form
high strength stainless steel fasteners. These alloys, however, have
limitations. While the hardenable stainless grades have the strength needed
for intended applications, they have lacked either the formability or the
corrosion resistance of the non-hardenable alloys.
|
| Trinamet™ stainless steel heading wire and samples of fasteners. |
For this
reason, the need has grown for a somewhat universal stainless heading alloy
with a strong combination of three key characteristics – good corrosion
resistance, high heat-treated hardness and cold formability.
When high
strength and corrosion resistance in industrial environments is needed,
fastener manufacturers have had to consider or resort to a variety of more
expensive and complicated options. These options include using galvanized
carbon steel fasteners, welding a high-hardness tip onto a stainless Type
304 screw
for drilling, machining fasteners from 17Cr-4Ni stainless bar, and using A-286
alloy, an
expensive high-nickel grade.
The problem
of under-performance pervades all categories of stainless steel. The austenitic
stainless Type 304, for example, provides good corrosion resistance and cold
formability, but cannot be hardened by heat treatment
Martensitic
stainless Type 410
offers good cold formability and high strength/hardness after heat treatment,
but lags in corrosion resistance. Martensitic precipitation-hardenable (PH) Custom
630 stainless (17Cr-4Ni) has high strength/heat treated hardness and good corrosion
resistance, but cannot be readily cold formed.
Potential Solution
In an
effort to provide an improved combination of all three critical properties for
the fastener industry, Carpenter’s Research and Development laboratory has
developed a new cold heading alloy known as Trinamet™ stainless, a hardenable martensitic
stainless steel that successfully combines corrosion resistance, good cold
formability and heat-treated hardness up to 53 HRC hardness.
The nominal
compositions of Trinamet stainless and other alloys commonly used for cold
heading can be seen in Fig. 1. Stainless Type 410 was used as a
baseline material for cold forming characteristics in operations such as
heading, thread rolling, slotting, extrusion, drawing, and flattening.
Compared to Type 410, Trinamet has higher carbon for higher hardness capability
for use in self-drilling applications and, higher chromium plus a molybdenum
addition for improved resistance in humidity and salt environments. Copper is
added to prevent excessive ferrite in the microstructure.
In the fully
heat treated condition, the newly developed alloy can achieve a hardness of
50-53 HRC, which is higher than the 45HRC max of hardenable 17Cr-4Ni
stainless
alloy and much higher than a non-hardenable alloy, like Type 304.
Trinamet
stainless heading wire can be considered for a variety of fastener applications
including sheet metal screws, self drilling construction fasteners and various
other bolts and fasteners exposed to typical industrial environments.
Cold
Formability
Trinamet
stainless can also be readily cold formed. It has a room temperature ultimate
tensile strength (UTS) of 90 to 110 ksi (620 to 758 MPa), and work hardens at a
rate of 1.1 ksi (7.6 MPa) percent cold reduction, which is comparable to the
rate for Type 410 stainless.
The new
stainless alloy offers a significant advantage over 17Cr-4Ni stainless. While
both have similar corrosion resistance in industrial environments, Trinamet
stainless has higher hardness. More important, Trinamet stainless can be cold
formed, while 17Cr-4Ni stainless is very difficult to cold form. This
characteristic can be very important to shops that are currently machining
17Cr-4Ni. Here an opportunity exists to move from an expensive machined part to
a less expensive cold headed part by switching to the more easily cold formed
Trinamet stainless.
Based on
existing data, the cold formability of most popular cold headed stainless
steels is compared in Fig. 1 in terms of “excellent,” “good” or
“moderate.”
Corrosion
Resistance
Laboratory
tests have shown Trinamet stainless to have corrosion resistance similar to
Type 304 stainless and better than that of Types 410, 420 and 440 stainless in 5% neutral
salt spray for 200 hours (ASTM B117). Fig. 1 rates the relative
resistance of most popular cold heading stainless alloys to salt spray in terms
of “good” or “moderate.”
For maximum
corrosion resistance, parts must be free of scale, foreign particles, free iron
and surface imperfections that can trap foreign material, thus contribute to
pitting and crevice corrosion, especially in the presence of chlorides.
Finished parts should always be passivated.
Mechanical
Properties
The
mechanical properties of Trinamet stainless are compared in Fig. 2 with
those of stainless Type 410 and stainless Type 420, alloys commonly used for
cold heading stainless parts, after hardening at 1900ºF (1038ºC) for 30 minutes
and tempering at 400ºF (204ºC) for two hours. Values are provided for relative
yield strength, ultimate tensile strength, percent elongation, percent
reduction of area and Rockwell C hardness.
A full
range of mechanical properties for Trinamet stainless can be seen in Fig. 3,
which provides values for each attribute at various tempering temperatures.
Typical
mechanical properties for a larger group of stainless steels used by the
fastener industry are presented in Fig. 1. Data indicates, for each grade, the
typical annealed UTS at room temperature and maximum hardness when fully heat
treated.
Heat
Treatment
Careful heat
treatment is key to obtaining the best combination of properties. Special
annealing is necessary to achieve maximum softness for cold forming. The alloy
can achieve an ultimate tensile strength of less than 100 ksi when fully
annealed. Hardening of finished parts, typically to 50-53 HRC, is essential for
corrosion resistance.
For best
heat treating results, finished parts should be cleaned, then hardened at
1900ºF (1038ºC) for ½ hr. in a vacuum or inert atmosphere to prevent oxidation;
quenched rapidly, then tempered at 350 to 600ºF (177 to 315ºC) for two hours
and air cooled to increase ductility.
Tempering
curves through specific temperature ranges, in Fig. 4, show relative
effect of tempering on ultimate tensile strength, yield strength and elongation.
Trinamet is not recommended for elevated temperature
applications since corrosion resistance and toughness would be reduced if the
alloy is heated above about 700/800ºF (371/427ºC) after hardening and
tempering, as recommended.
Selection
Guide
In the
universe of cold headable stainless steels, there are a select number of
reliable alloys that have proven useful for cold forming fastener applications
because of each alloy’s distinctive properties. To simplify selection of the
best and most cost-effective alloy for a given application, Carpenter has
developed a technique known as the Selectaloy® method.
This method
positions the new Trinamet alloy in the Selectaloy Diagram (Fig. 5),
thus indicating how its key properties relate to those of other stainless
steels commonly used.
The
selection diagram shows 22 popular stainless and high temperature alloys
positioned according to their relative corrosion resistance and headability.
Corrosion resistance, expressed on the horizontal axis, increases from left to
right. Headability, reflected along the vertical axis, increases from the
bottom up. Where two different alloys occupy the same oval, their corrosion
resistance and headability are considered similar.
When
considering a stainless steel for a given application, it is best to start with
stainless Type 410.
It is a simple and very popular martensitic alloy, with 12% chromium, that is
hardenable by heat treatment.
Fig. 5 —
Carpenter Selectaloy® diagram showing relative corrosion resistance and cold
formability of Trinamet™ stainless when compared with 21 other popular heading
alloys.
Contact Carpenter technical staff for further assistance
in selecting an alloy for a specific application.
The diagram
indicates that stainless Type 410 has a high level of cold formability, but
along with stainless Type 440C and Pyromet® Alloy 882, has the least amount
of corrosion resistance of all alloys depicted.
Trinamet
stainless, to the right of Type
410 stainless, has the same high level of headability as that for Type 410
stainless, but with increased corrosion resistance. Only stainless Type
409Cb and
stainless Type 430 exhibit better headability than Trinamet stainless at the same level
of corrosion resistance.
At the
right lower end of the diagram, Waspaloy, used frequently for aerospace fasteners, has
best corrosion resistance of any stainless in the diagram. The downside: it is
also the most difficult to cold form.
Alloy
Forms
Trinamet
stainless is available as mill-annealed STARR Wire® or cold drawn wire with
standard wire coatings. Wire is offered from 0.060"-1" (1.524 - 25.4
mm) coil. Mill annealing is designed to achieve maximum softness for
formability and obtain the best combination of properties possible.
For more
information about Trinamet stainless or other heading alloys, contact Carpenter
Technology Corporation, P. O. Box
14662, Wyomissing, PA, 19612-4662, USA, or
visit Carpenter’s website at www.cartech.com or
send e-mail to service@cartech.com or phone (610) 208-2000.
Registered
trademarks are property of CRS
Holdings, Inc., a subsidiary of Carpenter Technology Corporation. Selectaloy
arrows & ovals design reg. U.S. Pat. & Tm. Office.
