This week, we answer a question that is frequently asked by not just food industry professionals, but by people in several different industries including mining, recycling, agriculture, and so on...
Q: "Is stainless steel magnetic?"
A: There is no straight 'yes' or 'no' answer to this question. The magnetic susceptibility of stainless steel is dependent on a number of factors including its fundamental structure, the concentration of iron, and whether or not it has become 'work-hardened'.
Strictly speaking, austenitic steels such as 304 and 316 in their annealed condition are not magnetic - unless, they have been work-hardened. In contrast, steels with high levels of ferrite in their microstructure, such as 409 or 420, are strongly attracted to magnets.
Most equipment in food processing plants are made of 304 or 316 grade SS, so it's no surprise that when clients discover that small stones and fine stainless steel pieces are one of the most frequent contaminants extracted by magnetic separators, they ask the question: why are small pieces of stainless steel attracted to the magnets?
The process of 'work-hardening' can also be described as 'mechanical fatiguing'. Typically, a food processing plant has several stainless steel machines/pieces of equipment that the product must pass through. Thousands of moving parts wear against each other and can result in fine shards of SS fragments entering the product stream. Because these SS contaminants have become mechanically fatigued/work-hardened, they are now weakly-/ferro-magnetic and can be separated by suitable magnetic separation equipment. The degree of magnetic susceptibility of work-hardened stainless steel is dependent on the degree of work-hardening, the grade of the stainless steel, and the size of the SS fragment.
Work-hardened stainless steel poses a serious potential contamination risk in food processing, and as these contaminants can be extremely tiny (almost dust-like in size), it is vital to ensure that maximum foreign metal contamination control is achieved. It is recommended that both metal detectors and magnetic separators are employed to achieve this as if the stainless steel contaminant has a size and weight that outweighs its magnetic susceptibility in the surrounding conditions (product, speed, fragment size, etc), a metal detector needs to be relied on. However, if the fragment is tiny (less than 3 mm) there is a strong possibility the metal detector could miss the fragment and therefore high-intensity magnetic separation equipment is necessary to be installed upstream.
Like with stainless steel, some stones fragments are magnetic. There are a wide variety of stones, including blue metal, that are magnetic and their magnetic susceptibility is dependent upon the degree of iron content in the stone.
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