Why Choose Stainless Steel Spring Wire?
Reasons to Choose Stainless Wire
Resistance to corrosion
Resistance to temperature
Possesses good resistance to abrasion and wear
300 Series Spring Wire Series
- AKA the “18-8” grades of stainless. 18-8 stands for the chromium and nickel contents which provide the steel with excellent corrosion resistant properties
- Considered Austenitic stainless grades: typically cannot be hardened by heat treatment. Instead, they are hardened by cold working or drawing
- PH stands for precipitation-hardening. Parts gain strength after being heat treated at 900 degrees for I hour. This allows the spring maker to coil the spring in a softer state (Condition C) and then heat treat the springs to gain strength (Condition CH 900)
- Not considered to be 18-8 stainless due to the lower chrome and nickel, HOWEVER…
- 17-7PH has the addition of aluminum which precipitates out of the steel during the heat treat process, giving the steel:
o additional strength
o potential gain in tensile strength of 30,000+ PSI
Grades 304, 321, 347
- Austenitic (Cr-Ni) grades of stainless: Cannot be hardened by heat treat. Instead, they are hardened by cold working or drawing
- Uses: Pool cover springs, non-invasive surgical devices, clips, springs, fasteners
400 Series Stainless Spring Wire
Grades 410, 420, 430
- Martensitic (Cr) grades of stainless: can be hardened by heat treatment
- Normally used in the annealed condition due to being easily formed
- Compared to the 300 Series: cheapest grades of stainless with less strength and inferior corrosion properties
- Uses: applications such as fasteners, lock wire, springs, clamps, clips, wire-form
|Max Strength KSI
|Widely used, high strength, and good corrosion properties. Used for springs & wire forms.
|Typically softer than T-302. Easier to form. Used in annealed condition for fasteners & cold heading applications. Some 302 may meet chemistry of 304 but not vice versa. Tempered, 304 is used for springs & wire forms.
|Superior corrosion properties when compared to 302 & 304 due to addition of Mo, however, lesser strength. Used in spring form for medical devices.
|17-7 PH Cond C gains strength after heat treat 1 hr at 900 degrees. Same corrosion properties as 302.
Coatings Available for Stainless Wire
Coatings are put on wire for ease of drawing the wire but also to provide lubricant for the spring coilers.
- Phosphate™: Gibbs Interwire trade name for soap drawn stainless
- Non-metallic coating
- Most common and least expensive coating available
- How it is applied – Soap coating is a two-step process:
- First, a precoat is put on the wire in the annealed condition. Drawing soaps then adhere to the precoat during the drawing stage. Thus, the coating is composed of pre-coat and drawing soaps.
- Note: Each mill uses different pre-coats and drawing soaps
- Metallic premium coating
- Higher cost than soap coated
- Provides excellent lubricant for the spring coiler. A very consistent coating.
- Can be drawn in soaps or oil
- Soap-Drawn Nickel: provides more lubrication as soap will adhere to the nickel during the drawing process
- Oil-Drawn Nickel: provides less lubricant but cleaner material (no soap residue)
- How it works –
- Nickel is put on the process wire (annealed condition) electrolytically and then drawn providing excellent adherence to the surface of the wire
- Note: not all wire mills have the capability to provide this type of coating
- Very little lubricant
- Not typically recommended for spring coiling
- Used mainly where a bright wire is needed for aesthetics (luster) and/or a functional bright finish (when surface finish, Ra, RMS, roughness can affect the performance of the end product)
- Example Application: Bright Finish is standard in medical wire
- How it works –
- Wire is drawn in diamond dies and oil to provide the bright, lustrous finish
- Note: Generally finer diameters (.030 & below) are requested in this finish in the spring temper condition
Other Stainless Wire Info:
Operating temperatures for the 300 series are typically 400-450 degrees.
If higher operating temperatures are required, we suggest 17-7PH (550-
Have even higher temperature requirements? Go with superalloys such as Inconel X750 or Inconel 600.
Stainless wire is drawn from the annealed state. Annealing is a process where the wire is transformed during heat treat.
• After annealing, wire is considered soft in the tensile range of 80-120,000 PSI. The wire is then drawn to a particular size
• During the drawing process, the wire hardens due to cold work and gains strength
How it works – Example: A mill would like to result in wire .040” diameter in the spring temper condition. In order to achieve the diameter and desired temper, each finish size of material has a specific drawing process. This process may have to be adjusted slightly depending on the exact chemistry of the material involved. The goal is to meet the diameter tolerance per the ASTM A313
Typical Process for Tempering:
- The manufacturing mill would start with a larger diameter wire than desired in the soft or annealed condition
- It would then be drawn through a series of dies to reduce the diameter but will gain strength during the reduction process. However, it can only be drawn so far before becoming too hard or brittle
- In this example let’s say we draw the .250” to .100”. The material is now cold worked (drawing process) and at .100” becomes very hard, too hard to draw any further or it will become brittle. But we still need to get it down to .040”
- At .100” in the hard state we must again anneal to soften. During the annealing process the wire will be cleaned before annealing. Pre-coating will be put on after annealing
- After the annealing process is completed the material is again soft at .100”, pre-coated, and now ready to draw down to .040. Now the wire is drawn again through a series of dies, reducing it a little through each die (reduction in area) until it gets to its finished size of .040”. At that point, it will be within its tensile strength requirement and be ready to sell to spring customers
Corrosion properties of stainless steel are attributable to a very thin, invisible oxide film that completely covers the surfaces of the parts and prevents corrosion from taking place. Passivation removes any surface contamination, usually iron so that this film layer is not interrupted.
• Passivation enhances the formation of the oxide film for maximum corrosion resistance
• Passivation removes metallic and non-metallic coatings
• How it works – done by immersing the parts in a nitric acid solution, normally for 30 minutes
The process of heating material to a suitable temperature, holding long enough to reduce residual stresses and then cooling slowly enough to minimize the development of new residual stresses.
- Often suggested for stainless finished parts such as springs after the coiling process
- How it works – During the spring coiling process, additional stresses are introduced into the wire. Heat treatment post coiling relieves the stress
- Suggested Stress Relief:
o 300 Series = 700 degrees for one hour
- 17- 7PH = 900 degrees for one hour
Common Specifications for Stainless
- ASTM A 313
- Most common specification Gibbs Interwire orders to
- Covers all grades that we order, T-302/304 and T-316
- Grade T302/304
- ASTM A313 Type 631
- Type 631 covers 17-7 PH
- Also covers 17-7 PH