Trends for stainless steel tube in automotive applications

Will tubular automotive components now fabricated from carbon steel and steel alloys be fabricated from stainless steel in the future? Pictured here are a water pump/coolant tube, coolant bypass tube, water outlet tube, and hose assembly for an engine coolant system; transmission oil level tube; oil fill tube; heat/coolant air management components; and seat components.

Demand for stainless steel reached 24.6 million metric tons (mmt) in 2004, an increase of 7.5 percent over the previous year, according to the International Stainless Steel Forum. Evidence that stainless steel has potential as a material for automotive components—for its high strength-to-weight ratio for overall weight reduction, good dent performance, corrosion resistance, and formability—was presented by ISSF members at the SAE International™ 2004 SAE World Congress in Detroit.

Stainless steel already is being used in automotive exhaust systems. In the early 1990s, exhaust systems in new cars were made primarily of carbon steel and coated carbon steel. Today most new cars come equipped with stainless steel exhaust systems because of increased durability requirements. About 45 to 50 pounds of the stainless steel in automotive applications are in the exhaust systems, according to a technical paper on stainless steel automotive components presented at the conference.¹

In addition, stainless steel currently is used in the fabrication of restraint systems, fuel and brake components, and bus and truck trailer frames, according to the report.

Additional stainless steel components under development or consideration by vehicle manufacturers include engine cradles, intrusion beams, fuel tanks, tailgates and liftgates, bumpers and bumper beams, skid plates, crash-sensitive structural members, cylinder head gaskets, suspension and energy-absorption components, and wheels and wheel covers, according to the ISSF.

Stainless—The Other Light Meat

Over the past decade metal producers and researchers have investigated applications for stainless steel in automotive components to determine potential advantages of forming them from stainless steel instead of carbon steel and aluminum, according to the report.

In general, stainless steel costs more than aluminum and carbon steel* and so may not appear to be a cost-effective material, the report stated. However, when stainless steel's high strength-to-weight ratio is considered, it may be possible for lighter, thinner-gauge stainless steel to be substituted for a thicker-gauge carbon steel or high-strength (6000 series) aluminum alloy in automotive structural applications. In addition, stainless steel's potentially increased durability may make it a cost-effective material over the life cycle of the product, according to the report.

The largest weight savings and most cost-effective use of material can be achieved when stainless steel's properties are accounted for in the initial design, according to the report. Some automo-tive components' shapes and how they are fabricated should be modified to take advantage of stainless steel's weight reduction potential, the report's authors said.

For example, to take advantage of stainless steel's formability, some components would be better fabricated from hydroformed tube, the report indicated.

Part Redesigned for Hydroforming, Stainless

To calculate the weight reduction potential of stainless steel in automotive body structures responsible for energy absorption, such as the side members, a test case comparison was performed, and its results were detailed in another report presented at the conference.²

Tube Hydroforming and Stainless Steel "The primary motivation that drives the popularity of tube hydroforming, particularly in the automotive structural industry, is the anticipation of significant savings, i.e., weight, part numbers, etc., compared to established methods such as stamping ... The tube for hydroforming of the engine cradle was firstly by roll forming sheet in a five-step press brake bending and then laser welding." —Kwang-yuk Kim, Young-Hwan Kim, Yong-deuk Lee, Sung-ho Park, and Woosik Lee, in proceedings from "Application Study of an Automotive Structural Part With Nitrogen-alloyed High-strength Austenitic Stainless Steel" Figure 1 A carbon steel side structure was refabricated from a lighter, thinner-gauge stainless steel—first by stamping, and then by hydroforming. Simply substituting stainless steel for carbon steel offered a maximum weight reduction of 28.2 percent, while tube hydroforming it from stainless steel offered a maximum weight reduction of 33.8 percent.

First a carbon steel side structure (see Figure 1) was refabricated from a lighter, thinner-gauge stainless steel without modifying its shape. In a second analysis, the side member was refabricated from stainless steel, but also redesigned to be hydroformed to exploit stainless steel's favorable formability properties.

The carbon steel side member had a multipart, spot-welded design comprising three main deep-drawn parts and one reinforcement part. The overall weight of the side member without the connecting parts was 62 lbs. (28.16 kg).

Substituting 2B, type C600, and C1000 stainless steels alone resulted in a 9.3 percent, 14 percent, and 28.2 percent weight reduction, respectively, while retaining the same energy absorption.

When the side member was redesigned to be hydroformed, the 2B, type C600, and C1000 stainless steels resulted in a 19.4 percent, 30.2 percent, and 33.8 percent weight reduction, respectively. But a formability study showed that slight design modifications were necessary to fabricate the side member from stainless steel in cold-worked-condition C1000. The test showed that the gauges could be reduced without sacrificing crash performance.

The side member's maximum weight reduction can be achieved only with a redesign that takes advantage of stainless steel's formability capacity using hydroforming, the report concluded. Another advantage hydroforming offers is that the number of parts needed to form the side member can be reduced from four to one, eliminating the need to weld them. Because flanges are no longer needed, the closed beam cross section can be enlarged, and the profile can be designed straighter, the report stated.

A formability study showed that the new side member design could not be hydroformed with carbon steel. A simulation with the reverse method was done to determine if the new member design could be hydroformed from other high-strength carbon steels. The study showed the hydroformed design cannot be manufactured with high-strength carbon steel; therefore, stainless steel is required to achieve the weight savings for the side member.

In addition, hydroforming the side member offered improved crash performance. The bent shape of the side member constructed of carbon steel promotes buckling instead of crushing in frontal crashes, and this was confirmed in a crash simulation. In safety tests, crushing showed better results than buckling.

Notes
1. Gagan Tandon, John Tack, and Sanjaya Fonseka, "Lightweight Tailgates With Stainless Steel," in proceedings from 2004 SAE World Congress, sponsored by SAE International, Detroit, March 8-11, 2004.
2. Flavio Friesen and Pierre-Jean Cunat, "Application of Stainless Steel in Crash Structures of Vehicles," in proceedings from 2004 SAE World Congress, sponsored by SAE International, Detroit, March 8-11, 2004. International Stainless Steel Forum, www.worldstainless.org
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