Automotive engineers continue to look for ways to make vehicles lighter, cut fuel consumption and associated emissions, and improve performance and handling. For example, it’s estimated that emissions decrease 6.9% for every 10% of weight removed. So, engineers are turning to aluminum, magnesium and titanium alloys, composites, and advanced high-strength steels (AHSS) including martensitic stainless steels (MSS).
MSS combines strength, toughness, and corrosion resistance with ease of forming. New developments in welding and thermal processing are making MSS a cost-effective alternative to the materials engineers have been using to reduce weight.
Stainless, or corrosion-resistant, steels are iron-base alloys with a minimum 10.5% chromium content. Chromium promotes the development of an invisible, adherent, self-healing oxide surface film that staves off corrosion.
Stainless steels are commonly divided into five groups, classified by their microstructure at room temperature: austenitic, ferritic, duplex, precipitation hardenable, and martensitic. Various alloying elements added to basic iron-chromium-carbon and iron-chromium-nickel formulations control microstructures and properties. There are over 150 grades of stainless steel. Austenitic stainless steels such as types 304 and 316 are the most widely used, with roughly 18% chromium and 8% nickel content by weight.
MSS are ferromagnetic alloys in which chromium and carbon are the primary alloying elements. The most common MSS grade is type 410 which contains up to 0.15% carbon, 1% manganese, 0.03% sulfur, 0.04% phosphorous, 1% silicon, and between 11.5 and 13.5% chromium.
The increased carbon content and chemical makeup of MSS forces the atoms into a distorted body-centered cubic or body-centered tetragonal martensitic crystal structure when the alloys are hardened.
MSS strengths vary depending on alloy content and heat treatment; ultimate tensile strength ranges from 500 to 2,000MPa. In general, MSS’ corrosion resistance is not as good as that of other stainless steels, due to lower chromium content and an absence of nickel.
However, the low-alloy content also means lower cost and less price volatility than conventional stainless grades. MSS are generally selected for applications that call for strength, wear resistance, and corrosion resistance under ambient atmospheric conditions.
MSS’ mechanical properties, like high specific strength (UTS/density) let automotive engineers save weight by using thinner parts while maintaining or exceeding baseline strength. Looked at another way, MSS components satisfy stringent crash and safety requirements without adding weight.
A thinner-walled, larger-diameter MSS tube can be both stiffer and lighter than a conventional steel tube. However, designers should take care to maintain packaging constraints and avoid extremely thin-walled sections that may introduce new failure modes such as localized shell buckling.
Working with welds
To get the target strength and wear resistance — often measured as hardness — MSS are formulated so processors can put them through a quench-and-temper (Q+T) heat treatment. This means giving them higher carbon content — up to 1.2 wt% in ultrahigh-strength MSS grades, compared to ferritic stainless steels like type 409 with only 0.03wt% carbon — while maintaining the same 10.5 to 18 wt% chromium content.
The extra carbon forces the microstructure to go from ferrite to austenite at high temperature, then morph into martensite upon cooling. MSS are considered air-hardenable because all but the thickest sections reach peak hardness during the quenching portion of their heat treatment in which ambient air, not oil or water, returns the steel to room temperature.
Formulating the steel for this type of heat treatment has historically meant MSS is the least weldable stainless steel. Welding heats and rapidly cools the fusion and heat-affected zones in a way that mimics a rapid air-cooling quench cycle. Whatever the state of the MSS before welding — annealed, hardened, or Q+T — welding leaves untempered martensite in the fusion and heat-affected zones.
Untempered martensite is hard and strong, but extremely brittle and crack-sensitive. Weld quality is better when material in and around the weld is slightly more ductile.
Some processors add austenitic weld-filler alloy to reduce weld hardness. The filler lets a smaller percentage of martensite remain along with more ductile microstructures.
Another approach is to heat-treat welded parts. Processors can perform secondary postweld heat treatments (PWHT), such as process annealing, to control weld heat-affected-one hardness. Preheating is another way to reduce hardness; it slows the cooling rate and cuts the percentage of martensite.
Preheating MSS according to the American Society of Materials (ASM) welding guidelines means bringing the entire part up to 350°F. PWHT requires holding the welded area at temperatures over 1,100°F for several hours. These methods are neither cost nor time effective at high production rates, such as in the continuous production of seam-welded tubing where welding speeds exceed 20 fpm.
Another option is induction heat treatment, whereby parts heat rapidly from noncontact electromagnetic energy. A high-frequency alternating current passed through an induction coil near the part to be heated induces a corresponding current and resistive heating in the part.
Induction coils and power supplies can be tailored to heat parts of varying geometry and thickness, providing precise control of temperature throughout the part. Induction heat treatment’s rapid heating rate — up to several hundred degrees per second — and compact implementation can control weld cooling and perform PWHT without requiring a separate production step.
A recently developed weld-cooling control method uses induction to transform the microstructure of the MSS weld and heat-affected zones into tempered martensite, ferrite, and very fine carbides. The resulting metal has more ductility and toughness, less chance of hydrogen-induced cracking, and only slightly reduced strength.
Treated welds more reliably tolerate bending, flanging, hydroforming, and other common secondary operations without off-line heat treatment. Components processed with in-line weld-cooling control can go into service as-is.
Getting MSS-welding concerns out of the way lets processors focus on forming metals into components and assemblies. Processors can form MSS in complex, low-force stamping dies while the steel is in its softer annealed condition. MSS annealed yield strengths are typically under 350 MPa, similar to those of high-strength, low-alloy steels used extensively in automotive body structures
After forming, processors heat treat parts to get full strength. Q + T treatments’ gentle air-cooling cycles get parts to full hardness without excessive distortion or quench cracking.
Many processors use continuous furnace lines, such as mesh-belt or roller-hearth furnaces, to heat treat MSS. Equipment designed for bright annealing austenitic steels — heat treatment under reducing atmospheres to prevent oxidation — has similar temperatures, speeds, and throughputs to those required for bright hardening MSS.
The same postforming heat-treatment cycle lends itself to brazing and joining of MSS components without an extra step. Additionally, processors may use controlled atmospheres and tailored furnace-exit temperatures to get the right oxide thicknesses for adhesion with subsequent coating treatments.
Other ways to strengthen MSS include selective induction heating and hot stamping. Both methods have shown less surface scaling than conventional boron-treated steels, due to MSS’ protective chromium oxide surface layer. These techniques also let processors selectively heat MSS parts to fine-tune strengths and ductility within the same part.
Regardless of the thermal-processing method, the resulting hardened microstructure can be reset to a uniform, homogenous condition throughout, free from residual forming stresses. A standard e-coat baking cycle, or a final low temperature tempering process ensures a tough, resilient component.