Professional Steel Durability #1 – Use and Storage in the kitchen

When a consumer purchases a cookware utensil, such as a frying pan, there are many factors they might consider before doing so. One of these naturally would be the price, and another might be the aesthetic appearance of the frying pan. Another important consideration would be the durability of the pan; how long the consumer can use it before they would have to buy a replacement. Durability is especially important for utensils, such as frying pans, which are used on an almost daily basis. The consumer would want to know, for instance, whether the pan would warp after only little use or, if there was a coating on it, whether the non-stick release would expire quickly.

The above-mentioned are all legitimate concerns for the durability of a frying pan, but what a consumer might also like to contemplate is how durable their frying pan is when they are not cooking with it. For example, how resistant would the pan be to deformation if accidently dropped? Likewise, if the pan is stacked in a cupboard, with many other pots and pans resting above it, would it dent where the most pressure is applied? A bad dent or deformation might ruin the pan’s appearance, and a pan so deformed that it cannot sit flat on a cooking surface would be useless for cooking.

A frying pan falls to the floorPots and pans stacked in a cupboard

One way to examine whether a frying pan is durable enough to withstand the pressures of life in a kitchen would be to measure the hardness of the metal it is made of. Hardness is difficult to define in a short, simple sentence, but there are two generally agreed definitions:

  1. A measure of resistance to deformation (e.g. the pan being dropped on the kitchen floor).
  2. A measure of resistance to penetration (e.g. other pans or kitchen equipment resting on it).

One of the first methods ever developed for testing the hardness of a metal, and still often used today, is the Brinell Hardness Testing Method. Brinell hardness is determined by forcing a hard steel or carbide sphere of a specified diameter under a specified load into the surface of a material and measuring the diameter of the indentation left after the test. The Brinell hardness number, or simply the Brinell number, is obtained by dividing the load used, in kilograms, by the actual surface area of the indentation, in square millimeters.

The steps for the Brinell Hardness Testing Method are as follows:

Step 1 = The cookware sample is placed on the anvil

Step 2 = The penetrator contacts and indents the cookware sample

Step 3 = The penetrator is released

Step 4 = A microscope with a calibrated lens is used to measure the diameter of the dent

Brinell Hardness Test

As you can see from the table below, cold-rolled steel (the material used for Professional Steel) has a higher BHN value than aluminum. This means a Professional Steel pan is much more resistant than an aluminium pan to deformation when dropped on the floor, and to penetration by a heavy load resting on it.

BHN Values

A cracked cast iron panOn the other hand, as the table above demonstrates, there is another material which has a higher hardness value than either aluminium or Professional Steel, and that is cast iron. Cast iron undoubtedly has particularly strong compressive strength compared to most other metals used in cookware. Items stacked on a cast iron pan in a cupboard would have to be particularly heavy and positioned at a particular angle to leave a lasting dent. However, when considering which pan is the most durable for use in a kitchen, cast iron’s high hardness and compressive strength can be a bit misleading. This is because, although a dropped cast iron pan might not dent, something much worse could happen: it might fracture. Cast iron is very hard, with excellent compressive strength, but is also very brittle. It cannot absorb energy very well, and so has poor toughness. A sudden impact may not dent a cast iron pan as it would an aluminium pan, but it could cause a large crack in its structure.

One way to determine the amount of energy absorbed by a material during fracture is the Charpy impact test. The apparatus of this test consists of a pendulum of known mass and length that is dropped from a known height to impact a notched specimen of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after the fracture (energy absorbed by the fracture event).

Charpy Impact Test

It should be emphasized that Charpy tests are qualitative; that is, the results can only be compared with each other or with a requirement in a specification – they cannot be used to calculate the fracture toughness of a weld or parent metal. However, in general, Professional Steel pans can withstand a much larger depth of impact than either cast iron (because of its brittleness) or aluminium (because of its lesser hardness value and strength).

For a better comparison of toughness between aluminium, cast iron and Professional Steel a stress-strain diagram can be used. Toughness is related to the area under the stress-strain curve. To be tough, a material should withstand both high stresses and high strains. Generally speaking, strength indicates how much force the material can support, while toughness indicates how much energy a material can absorb before rupturing.

The portion of the stress-strain curve between zero and proportional limit (the limit beyond which stressed material will not return to its original length when the load is released) is known as the elastic range. As long as both stress and strain increase at a constant rate, the stress-strain diagram will be linear, i.e. a straight line. As shown in the table below, the Modulus of Elasticity, which lies within the elastic range, for Professional Steel is greater than both cast iron and aluminium. This means that even if a Professional Steel pan is dented by another object leaning against it, the pan may still return to its original length after that object is removed. 

Modulus of Elasticity

Stress-strain diagrams have different shapes for different materials. Aluminium does not show the distinguishable proportional limit that Professional Steel does. An aluminium pan has a much smaller elastic range than Professional Steel, so it is far more likely to remain plastically deformed (dented) and not return to its original shape. Conversely, cast iron normally fractures very close to the proportional limit because it is so brittle. In the stress-strain diagram for Professional Steel, the straight line continues for longer before it reaches its proportional limit than that for cast iron. Beyond this point strain increases at a faster rate than stress and the modulus of elasticity no longer applies.

Aluminum and Cast Iron Stress Strain Diagram

Professional Steel Stress Strain Diagram

Summary

When compared to aluminum and cast iron, Professional Steel is neither the hardest, the strongest nor the most ductile. However, Professional Steel is the toughest of all these metals. In order to be tough, a material must be both strong and ductile. For example, brittle materials, such as cast iron, that are strong but have limited ductility, are not tough. If you were to accidently drop a cast iron pan then it could easily fracture, which would make it useless for cooking. Conversely, very ductile materials, such as aluminium, with low strengths are also not tough. As shown on the stress-strain diagram, an aluminium pan may not fracture as easily as cast iron pan but its elastic range is very limited and it will plastically deform (dent) relatively easily. A Professional Pan is not immune to being deformed; its hardness value is less than cast iron after all. However, even if it does deform there is still a chance that its high modulus of elasticity will enable it to return to its original shape once the load is removed.

 

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