Stress corrosion cracking is a common failure mechanism for stainless steels. It is a threat to stainless steels used for plastic molding but is not typically very widely understood in the plastics industry.
Stress corrosion cracking is defined as the formation of cracks due to the combination of tensile forces and a corrosive environment. The cracks will initiate at localized stressed areas and move along specific paths within the material, typically following the steel抯 grain boundaries. The time it takes to propagate the crack depends on the intensity of the tensile stresses, the concentration of the corrosive environment, and the temperature of the environment. Stress corrosion can occur even in mildly corrosive environments. The plastics molding industry uses stainless steels in order to combat the corrosive attack of specific polymers and corrosive by-products of the molding operation.
How can a mold made from stainless steel crack due to corrosion issues when corrosion resistance is the main purpose for using the stainless mold steel? The corrosive attack is not due to the molding operation or anything associated with the surface of the mold. The attack is originating within the mold, specifically the internal waterline network. Since the molding surface is not affected, and the real cause is internal, stress corrosion is not often looked at as a potential cause of the mold failure.
A tensile stress can generate cracking. In operation, all molds flex. The flexing of the mold will generate tensile stresses within the waterline network. Stresses will be higher at waterline intersections and threads. In threaded areas, the tensile forces created by plugs can be more than enough to generate stress corrosion cracking, even when the mold is not operating. The second cause of stress corrosion cracking is the corrosive environment. For stainless steels, the presence of chlorine is the main cause of stress corrosion cracking.
Chlorine is found in nearly all public water supplies. The concentration of chlorine depends on the guidelines set by local water suppliers, operating within government guidelines. Increasing the temperature of the environment will increase the corrosive attack. Since all plastics molding is done at elevated temperatures, the molding temperature can affect the rate of stress corrosion cracking.
A closure mold insert of 420 ESR material shows the part and the stress corrosion area. Stress corrosion can occur even in mildly corrosive environments and is not caused by the molding operation or the mold surface. Corrosion of stainless steel originates within the mold at the internal water-line network.
The presence of chlorine and tensile forces creates a chemical reaction between the chromium in the steel and the chlorine ions. The result is a loss of chromium in the steel by formation of chromium chloride. Chromium chloride is soluble in water, so once it is formed it dissolves into the cooling line water. This reaction will tend to occur at the highest chromium concentration in the steel. In the case of stainless mold steels, this typically occurs at the steel grain boundaries where chromium carbide deposits are found.
The amount of chromium carbide is dependent on the heat treatment of the steel, so how a mold is heat-treated can affect stress corrosion rates. Heat treatment is critical to ensuring optimum corrosion resistance of any stainless steel. Each supplier of stainless has guidelines for proper heat treatment. In our case, we work with heat-treatment suppliers to optimize processing of materials that originate from us. Many molders set up specific guidelines at each heat-treatment vendor for processing their mold steels.
The corrosive attack at the steel抯 grain boundaries results in fine cracks in the steel抯 matrix. The fine cracks are stress risers and result in an increase in the crack propagation. When the cracks progress far enough into the steel, the stresses of molding are enough to continue the crack migration to the outer areas of the mold.
The cracks that form are inter-granular and have a brittle appearance even if the material is ductile. It is this inter-granular appearance that often results in stress corrosion cracking being misinterpreted as a mechanical fracture instead of corrosion related. The photos on p. 24 show a closure mold insert made of 420 ESR material, along with the part and the stress corrosion area. Stress corrosion cracking was confirmed using a scanning electron microscope that showed the presence of chlorine in the corroded areas.
In summary, stress corrosion cracking is a real prospect that must be taken into consideration when using stainless mold steels. The devastating part of stress corrosion cracking is the fact that the problem is not seen or addressed until the mold is cracked through to the visible surface. In most cases, the mold is cracked beyond repair at this stage and will need to be replaced.
To prevent stress corrosion cracking it is necessary to control the conditions under which it occurs. Reducing tensile stresses in the mold and reducing the chlorine content inside the waterlines are the best ways to diminish the threat of stress corrosion cracking. All issues regarding water should be addressed with water experts. Some molders outsource their water treatment and maintenance and some monitor chlorine, conductivity, and ph (acid/base index) in-house. If a processor has never reviewed water quality, it would be advisable to consult an expert for advice.
Contact Information
Bohler-Uddeholm, Rolling Meadows, IL
Ed Severson
(847) 577-2220
ed.severson@bucorp.com (end)
