Spring Materials


Details

Springs are manufactured either by hot- or cold-working processes, depending upon the size of the material, the spring index, and the properties desired. In general, pre-hardened wire should not be used if D/d < 4 or if d > 1/4 in. Winding of the spring induces residual stresses in a coil spring. Quite frequently in spring manufacture, they are relieved, after winding, by a mild thermal treatment.

A great variety of spring materials are available to the designer, including plain carbon steels, alloy steels, and corrosion-resistant steels, as well as nonferrous materials such as phosphor bronze, spring brass, beryllium copper, and various nickel alloys. The following table lists the most commonly used steels:

Music wire
Oil quench and temper wire
Hard-drawn wire
Chrome-vanadium wire
Chrome-silicon wire

Spring materials may be compared by an examination of their tensile strengths; these vary so much with wire size that they cannot be specified until the wire size is known. The material and its processing also, of course, have an effect on tensile strength. It turns out that the graph of tensile strength versus wire diameter is almost a straight line for some materials when plotted on log-log axes. Writing the equation of this line as:

(Eq1)    
Sut =
A
d m

furnishes a good means of estimating minimum tensile strengths when the intercept A and the slope m of the line are known. Values of these constants have been worked out from recent data and are. In Eq1 when d is measured in millimeters, then A is in MPa · mmm and when d is measured in inches, then A is in kpsi · inm.

Although the torsional yield strength is needed to design the spring and to analyze the performance, spring materials customarily are tested only for tensile strength—perhaps because it is such an easy and economical test to make. A very rough estimate of the torsional yield strength can be obtained by assuming that the tensile yield strength is between 60 and 90 percent of the tensile strength. Then the distortion-energy theory can be employed to obtain the torsional yield strength (Sys = 0.577Sy). This approach results in the range:

0.35SutSsy ≤ 0.52Sut

for steels.

For wires listed in table, the maximum allowable shear stress in a spring can be seen in column 3. Music wire and hard-drawn steel spring wire have a low end of range Ssy = 0.45Sut. Valve spring wire, Cr-Va, Cr-Si, and other (not shown) hardened and tempered carbon and low-alloy steel wires as a group have Ssy ≥ 0.50Sut. Many nonferrous materials (not shown) as a group have Ssy ≥ 0.35Sut.


Spring Materials for Fatigue Applications

Since spring wire is primarily subjected to torsional stresses, maximum stress levels occur at the wire surface. As a result, material surface defects (i.e. seams, laps, pits, etc.) can dramatically reduce a spring's fatigue life. Knowing this, wire manufacturers have developed surface preparation methods to restrict the size of wire surface defects as it leaves the mill. Wire produced with these methods is rated for fatigue applications and is often referred to as "valve spring quality". Since these methods often involve costly processes, fatuge-rated spring wire is often significantly more expensive than its commercial-grade counterpart.

The two most popular materials for fatigue applications as of 2008 are Music Wire (ASTM A228) and Chrome-Silicon Valve Spring Quality (ASTM A877). At wire sizes below approximately 0.080" (2.0 mm), Music Wire offers higher tensile strength; however, Music Wire's maximu service temperature is less than that of Chrome-Silicon.


Materials for High Temperature Applications

As the temperature resistance increases, the material and processing cost typically increases significantly. Therefore, it is usually wise to select a material that provides resistance for the intended temperature range with minimal excess capability. The table below lists a variety of spring materials and their maximum service temperatures.

Wire TypeMaximum Temperature (°F)
Music Wire250
Hard Drawn Carbon250
Oil Tempered Carbon300
Chrome Vanadium425
Chrome Silicon475
302 Stainless500
17-7 PH600
NiCr A286950
Inconel 600700
Inconel X7501100


Difference Between Hard Drawn Wire and Music Wire

Both hard drawn wire and music wire gain their strength through cold drawing the wire from large diameter rod to its final size. There are three significant differences. First is the chemical composition of the wire. Music wire contains more carbon and less manganese than hard drawn wire. Additionally, the allowed levels of contaminants such as phosphorous and sulfur in music wire are more restrictive. The second key difference is in the wire's strength. Because of the additional carbon, music wire can be drawn to significantly higher tensile strengths than hard drawn wire. Finally, processing of music wire is done in a manner to provide a finished surface with smaller allowed defects than hard drawn wire. Since surface defects are one of the most common initiation sites for fatigue cracks in springs, smaller surface defects (and their corresponding reduction in stress concentration) enable music wire to be used in high cycle fatigue applications. Hard drawn wire is best suited to static or very low cycle service conditions.