Low-Temperature Hardening Treatment Of Stainless Steel Used for Automobiles Surface

Publish Time: 06/22 2021 Author: Site Editor Visit: 14906

Although austenitic stainless steel has been widely used due to its excellent corrosion resistance, its tribological properties are poor, especially its low abrasion resistance or adhesive wear performance, and has a tendency to fretting wear, which hinders the need for corrosion resistance and Application in the field of wear resistance. The stainless steel processing (S3P) line of Baldick in Germany provides a solution to increase the mechanical properties of the material without changing the corrosion resistance. The thermal chemical diffusion process forms a carbon-rich or nitrogen-rich S phase while avoiding the precipitation of carbides or nitrides. Typical applications include food, beverages, drugs, medical supplies, and chemical process equipment. However, this article will mainly introduce the application of stainless steel in the automotive industry.

Stainless steel has long been used as automotive structural parts or decorative parts. A typical example is the austenitic stainless steel front cover and stainless steel inserts of the Rolls-Royce Phantom. Among the stainless steels, ferritic stainless steel is mainly used in automobiles, because ferritic stainless steel can make the exhaust system work in a high temperature and corrosive environment. Parts such as exhaust manifolds, pipes and silencers are all made of AISI 409 stainless steel. More expensive stainless steels, such as duplex stainless steels, have been widely used in automobiles. Functional parts made of duplex stainless steels combine mechanical strength and extremely high corrosion resistance. The automotive industry is facing strict requirements for safety and efficiency, which is an excellent opportunity for the application of stainless steel materials.

The New Frontier Of Resistance To Mechanical Friction

Let's look at a specific example. An automotive engineering company produces sliding bearings. After applying S3P line processing, the performance of the produced sliding bearings is improved, which proves how S3P perfectly improves the mechanical properties of stainless steel. For solenoid valve sliding bearings, when unhardened stainless steel is used, the bearing fails after 240,000 operations, while when using diamond-like carbon coating (DLC) material, it fails after 74 million operations. When using stainless steel with low-temperature hardening of the surface, the bearing life far exceeds any previous record, and the bearing can still run after 560 million operations. Although DLC has very high sliding properties, its surface hardness far exceeds that of S phase, but S3P treatment can greatly improve the comprehensive performance of stainless steel parts such as corrosion resistance, hardness, toughness and fatigue strength.

More in-depth application areas

The key advantage of the 10-40μm diffusion layer formed under the condition of surface hardening at low temperature is to eliminate any possible delamination risk, which is also beneficial in terms of resistance to pressure shock and thermal shock. Although the hardness of the carbon-rich S phase in stainless steel exceeds 1000HV0.05, it still maintains plasticity during the rapid furnace loading process. These characteristics make it used in many places in the high-yield, high-tech automobile industry. The current application areas include internal combustion engines, power transmission, transmission and exhaust systems. S3P can harden even the smallest parts, which leads to cost optimization. The examples given below illustrate that these hardened stainless steels can meet more areas and reduce overall costs.

Eliminate scratches on power connections

The mechanical moving parts installed in the accessories of internal combustion engines suffer from relatively high corrosion. Salt on the road or driving near the coast increases the risk of corrosion damage to different mechanical parts. The high temperature in the engine compartment further increases the corrosion effect. When considering the use of corrosive biofuels inside the engine, mechanical parts are also subject to other forms of corrosion.

For power connectors, scratch resistance is particularly important. Compared with untreated surfaces, low-temperature carburized surfaces have obvious advantages. The test results of AISI 316L stainless steel in accordance with the ASTM G98 standard show that compared with the untreated surface, the scratch threshold stress value is greatly increased after treatment. The untreated stainless steel was scratched at 45.5MPa, while the carburized surface was not scratched at 842.5MPa. This value exceeds the yield strength of the material, so plastic deformation occurs.

Alternative coating

Due to the European Union’s Vehicle End-of-Life Directive or Restriction of Hazardous Substances (RoHS), the European automotive industry has begun to replace hard chrome coatings. The requirements for corrosion-resistant and wear-resistant parts have led to the development of this substitution in two directions. One is to use other types of coatings, such as electroless nickel plating, and the other is to refine the matrix material through a diffusion process. From the perspective of wear, the coating is more advantageous than the diffusion process for stainless steel. The thickness of the hardened layer of the coating can be thicker, but the S3P process can improve the corrosion resistance without the risk of incomplete coating.

Not requiring machining after processing is an advantage of the S3P process, thereby reducing the overall cost and realizing the possibility of processing complex geometries, sharp edges and internal structures, which is usually impossible for coatings. Indeed, due to the characteristics of the diffusion process, it is possible to treat large quantities of materials with S3P surface hardening at low temperature. During rapid furnace installation, the hardened zone deforms without cracks or delamination. This fault tolerance is a big advantage compared to coatings or other diffusion-based processes. Even the smallest parts can be uniformly hardened, which is another advantage of this process, which is impossible with other technologies. Surface hardening of holes with a diameter of 60μm is the unique ability of this process. Especially when subjected to high cycle bending fatigue, hard chromium and other coatings present a high risk of failure. Although studies claim that the coating has high mechanical strength and can improve fatigue resistance, when the specific stress level that is prone to crack initiation and propagation is exceeded, the transition zone from the soft layer to the hard coating is the weak point. This can lead to delamination of the coating or propagation of cracks into the soft layer, thereby affecting fatigue life.

Plain bearings without other seals

Another application area of surface hardening stainless steel is sliding bearings that do not require other seals, which can be used in high-pressure injection systems or pharmaceutical pumps. Linear drives also benefit from this technology. Due to the extremely low wear rate of metal-metal contact and the elimination of scratches at the same time, a simple structural design without polymer seal can be realized, as shown in Figure 3.

Improved spring fatigue performance

Due to the presence of carbon-rich S phase, stainless steel has a higher fatigue limit, the life cycle of the spring can be extended, and the possibility of failure is reduced. Depending on the size of the spring, the spring constant may also change. The smaller the size of the spring, the greater the effect of surface hardening treatment on the spring performance. The study of the carbon-rich S phase in AISI 304 stainless steel film found that the tensile strength can be increased from 600 MPa without treatment to more than 1500 MPa.

Repeatable and low cost

The low-temperature surface hardening treatment allows the stainless steel to improve its mechanical properties while maintaining good corrosion resistance. Better wear, abrasion, fatigue and cavitation properties make the surface low-temperature treated stainless steel widely used in the automotive industry. In order to meet the requirements of reliable parts and more efficient automobiles, surface-hardened stainless steel is a key material. The specific examples mentioned above, such as power connectors with higher fretting performance, stainless steel fuel accessories that will not block, springs with higher fatigue performance, and parts used for high-pressure valves, fully explain that the surface low-temperature hardening treatment can improve Various properties of stainless steel. In addition, it is important that this process achieve uniform, repeatable results. A breakdown or malfunction of a car while driving is a major accident for a brand car manufacturer, and this can be solved by low-temperature surface hardening technology. In addition to improving the process performance of hardened parts, economy is also important. S3P surface low-temperature hardening treatment can also process a large number of small-sized parts at one time to meet this goal.

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