New Hardenable Stainless Combines Corrosion Resistance, High Hardness and Cold Formability
The problem of underperformance pervades all categories of stainless steel. The austenitic stainless Type 304, for example, provides good corrosion resistance and cold formability, but it 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 it cannot be readily cold formed.
Potential SolutionIn 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.
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 it has 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 maximum of 45 HRC for the 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 FormabilityTrinamet 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) for each percent of cold reduction – comparable to the rate for Type 410 stainless steel.
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 manufacturers 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 ResistanceLaboratory 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). Figure 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 contributing to pitting and crevice corrosion, especially in the presence of chlorides. Finished parts should always be passivated.
Mechanical PropertiesThe 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 1900°F (1038°C) for 30 minutes and tempering at 400°F (204°C) 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 for each grade indicates the typical annealed UTS at room temperature and maximum hardness when fully heat treated.
Heat TreatmentCareful 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:
- Hardened at 1900°F (1038°C) for ½ hour in a vacuum or inert atmosphere to prevent oxidation
- Quenched rapidly
- Tempered at 350-600°F (177-315°C) for two hours
- Air cooled to increase ductility
Trinamet is not recommended for elevated-temperature applications since corrosion resistance and toughness would be reduced if the alloy is heated above about 700-800°F (371-427°C) after the recommended hardening and tempering.
Selection GuideIn 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 commonly used stainless steels.
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 12%-chromium martensitic alloy that is hardenable by heat treatment.
The diagram indicates that stainless Type 410 has a high level of cold formability but, along with stainless Type 440C and Pyromet 882 alloy, 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 the best corrosion resistance of any stainless in the diagram. The downside is that it is also the most difficult to cold form.
Alloy FormsTrinamet stainless is available as mill-annealed STARR Wire or cold-drawn wire with standard wire coatings. Wire is offered from 0.060-inch (1.524-mm) up to 1-inch (25.4-mm) coil. Mill annealing is designed to achieve maximum softness for formability and obtain the best combination of properties possible. IH
For more information about Trinamet stainless or other heading alloys, contact Carpenter Technology Corporation, P. O. Box 14662, Wyomissing, PA, 19612; tel: (610) 208-2000; e-mail: email@example.com; web: www.cartech.com. Author John Magee is specialist – Stainless Alloy R & D, and Tinsley Fehnel is product application metallurgist for Carpenter Technology Corporation, Wyomissing, Pa.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: fastener, cold heading, precipitation hardenable, passivated, austenitic stainless, martensitic stainless