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Properties of Steel | Understanding Material Properties

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Steel Characteristics & Properties

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Every type of steel has unique properties that affect how it performs. When finding a type of steel to specify its important to understand how the material properties will affect all aspects of your project.

Just with most things in life, there are trade offs when selecting a type of steel. This requires having a thorough understanding of the manufacturing, construction or assembly, and use of your product or project prior to selecting ideal steel properties.

Characteristics

Weldability

Weldability is a property of steel that greatly affects how easily it can be used in construction and fabrication. A steels weldability determines how easily a material can be welded. Materials with low weldability are likely to crack due to the local stresses caused from heating at the weld joint. A materials weldability is inversely correlated to the materials hardenability. This is because if a material is hardenable it will tend to harden during the welding process which can increase the brittleness and lead to cracking due to local thermal strain.

There are many different welding methods including Arc (aka stick) Welding, MIG Welding, TIG welding, Flux-Cored Arc Welding, Energy Beam Welding, Friction Welding, and more. Each welding method is used in different scenarios for different types of metals.

To weld steels with low weldability you can a use a heat treatment process which will increase the ductility of the material during the weld making it less susceptible to cracking. You may also need to relieve the residual stresses after the welding through another heat treatment process.

Hardenability

If your design is going to be required for cutting or need substantial wear resistance then the hardenability property of steel should be weighed in your decision.

A materials hardenability determines how easily the material can be hardened by thermal treatment. As hardenability increase, weldability decreases and vise versa. Steel with adequate or high hardenability can have hardness levels specified during the design phase. This is standard practice for tooling, and applications that require surface durability. Since hardness and ductility and inversely related, controlling the hardness of a material allows you to optimize the materials properties.

Hardenability can be affect by alloys, but is also dependent upon carbon content. Tool steel alloys which have exceptional hardenability also have high carbon content. Many steels can also only be surface hardened and not through hardened.

Machineability

If you are going to have to cut or remove material for your design then the machineability property of steel should play a role in your material selection.

Machineability is dependent upon a lot of factors. If a material is too hard it will reduce the tool life and dramatically increase the part costs. If a material is too ductile it can spring back after being cut leading to difficulty meeting tolerances. The most machinable metals are those with lower hardness and moderate ductility. To avoid wearing down tooling quickly, most metals are heat treated to the desired hardness after being machined. 01 Tool steel for instance is machined after being fully annealed to remove any residual stress and improve machinability. Once tool steel is machined it is then heat treated up to the desired hardness.

A materials ability to be work hardened also can reduce the machineability of a part as it deforms and hardens during manufacturing. This can lead to thermal build up in the machined part instead of the metal chips causing thermal distortions making meeting tolerances difficult. If cut rates and speeds aren’t properly adhered to it’s also possible for some metals to work harden to the point that they meet the same hardness as the tool leading to a dangerous tool failure. Metals such as stainless steel and high-temperature alloys are the most prone to work hardening and require extra care during machining.

A machinability rating system has been created which is based on a significant number of factors. The system uses 1212 steel as it’s benchmark 100% rating.

Material Machinability Chart

Workability (Bending / Forming)

If your design requires bending steel or if you can benefit from the low cost and high volume of stamping, then the workability property of steel will be critical for your project.

Workability affects how easily a material can be bent or formed. This is commonly done to form sheet metal or even steel plate into various shapes including anything from car panels to very large rolled steel tubes. Metals with high work-ability can be used in stamping without the need for expensive servo presses or can be easily formed into various shapes with tight bending radius’s.

Material properties including hardness and ductility have a large effect on workability. Higher strength metals such as high carbon steel have lower ductility making them far less workable compared to low carbon steel which has high ductility. In order to form metal you have to yield it making metals with a high yielding point and lower ductility less workable as they require more energy to bend and are prone to fracturing during bending. A materials stress strain curve can guide how much a material can be formed prior to failing.

Learn More About Stress Strain Curves

After a material has been worked it will retain residual stresses and have reduced ductility due to work hardening. If needed the material’s residual stresses can be relaxed by annealing the formed metal which removes residual stresses and returns ductility.

Workability can also be increased by heating the metal. This is refereed to as hot workability. As a metal is heated it’s ductility increases and the yield stress decreases which leads to dramatically increased workability. This can be used to hot form higher strength metals that would typically crack if cold formed.

Wear Resistance

If you’re making a cutting edge, a stamping die or something similar then the wear resistance property of steel will be dictate how long your tool can be used prior to failing.

Wear resistance is a materials resistance to surface material loss due to some form of mechanical action such as abrasion, erosion, adhesion, fatigue, or cavitation. Materials such as Diamond and Sapphire have extraordinarily high wear resistance which makes them ideal for use as gem stones that last a lifetime or used in demanding cutting tools. Surface hardness greatly affects the wear resistance of a material. The high surface hardness of a file allows it to wear down other metals of lower hardness without experiencing significant wear itself.

The hardness or wear resistance of metals is affected by the lattice geometry formed by the atoms of the metal. If atoms are able to move or dislocated within this lattice due to irregularities then the hardness of the metal is lower. When dislocations are prevented due to the lattice structure then the hardness of the metal increases delivering improved wear properties. When a metal is heat treated in order to increase the metals hardness the lattice structure is rearranged to form martensite in which the lattice structure is far less prone to slipping.

Learn More About Heat Treat Hardening

Corrosion Resistance

Corrosion resistance measures how well a material can withstand damage caused by oxidation or other chemical reactions. Metals have different levels of corrosion resistance.

Metals that are going to be exposed to rain, water, humidity, or anything else that can cause a metal’s surface to oxidize are vulnerable to corrosion damage. To protect against corrosion you can use stainless or galvanized steel, titanium, aluminum, weathering steel, or add and maintain a sealant layer such as paint.

Unless a metal is only exposed to vacuum, after enough time corrosion will occur. This is why corrosion prevention maintenance and monitoring is needed for any critical component. To determine maintenance recommendations you will want to calculate the corrosion rate.

Due to the high cost of stainless steel and aluminum most large scale civil projects today rely on weathering steel or sealants such as paint, or concrete cover to prevent corrosion damage.

While materials such as stainless steel, galvanized steel, weathering steel, titanium, or aluminum are highly corrosion resistant, they are not corrosion proof. Stainless steel, contains a very thin oxide layer which remains passive in the presence of corrosive elements. It is possible for the passive layer to break down exposing localized spots to corrosion. Galvanized steel provides corrosion resistance through a thin layer of zinc coating which bonds with the iron. Should the galvanized layer wear away the steel will become susceptible to corrosion again. Similarly weathering steel, titanium, or aluminum can all be affected by corrosion under certain situations. The best protection from corrosion is monitoring and maintenance.

Properties

The Stress Strain Curve

Yield Strength (Yield Stress or Point)

The yield strength of a material is the point at which a material begins to undergo a significant increase in the rate of strain in relation to stress. At this point ductile materials such as low carbon steel will begin to undergo significant deformation. An example of this is an overfilled room where the floor begins to deflect far more than what is typical.

Most designs will use yield strength as the design limit as once a material goes past the yield point it’s fatigue life becomes dramatically reduced. Some designs where remaining below the yield point of a material can add significant cost or that only require a limited number of uses may exceed the yield point and allow plastic deformation. To design a component for plastic deformation while meeting required cycle counts you will need to use more advanced analysis techniques such as nonlinear transient FEA which ASR Engineering provides frequently for our clients.

Springs are reliant on a very high yield strength which allows them to remain elastic and spring back to their original position after deforming.

Learn More About Our Analysis Services

Tensile Strength (Ultimate Stress)

The tensile or ultimate stress of a material is the point at which deflection will continue until fracture unless if the load is reduced. In other words this the amount of stress that will cause a material to fail with enough time. If you approach the tensile strength of a material you will either need to add reinforcement, increase the cross sectional area, switch to a higher strength material, or reduce the load.

Elongation

Elongation measures how much a material will stretch compared to its initial state prior to fracturing. This is communicated as a percentage of total elongation divided by the initial length. For instance a 1 inch long rubber band which can elongate to 2 inches prior to fracturing would have an elongation of 100% at fracture.

The more brittle a material is the less it will elongate prior to fracturing. Materials such as concrete or glass are extremely brittle and fracture or crack if they experience nearly any elongation. Metals however vary significantly in how much they can elongate prior to failure. For instance, alloy and low-carbon steels will typically elongate far more than high-carbon steels.

Hardness

The hardness of a material measures how much it will resist local plastic deformation due to mechanical indentation or abrasion. Hardness is especially important during manufacturing. Materials with high hardness are not able to be machined or formed similarly to materials with lower hardness. Typically metals will be hardened through a heat treatment process after being formed or machined in order to meet required specifications without dramatically increasing manufacturing costs.

While there are multiple hardness scales and types, the most popular for machining is the Rockwell scale. The Rockwell test measures the depth of penetration of an indenter under a large load and compares it to the penetration by a preload. Unlike other hardness tests, the Rockwell test is considered nondestructive. There are three Rockwell hardness scales including HRA, HRB, and HRC which are selected depending on which best represents the materials hardness with HRC representing the hardest materials.

Very hard steel such as chisels, high quality knives, tools, and files have hardness of between HRC 55-66. Meanwhile non-heat treated steel such as A36 doesn’t even use the higher HRC scale and has a hardness of only HRB 67-83 or HRC N/A-2 (HRB 67 doesn’t overlap with the HRC scale).

If you ever wondered why the quality of a knife edge or cutting tool can vary so much it is because of hardness. When you pay for a top quality knife or tool a lot of what you are paying for is the added work and difficulty it takes to acquire the desired hardness which can last without dulling for far longer than low quality competitors.

Types of Steel & Their Properties

Carbon Steel

There are three types of carbon steel. Low-carbon, medium-carbon, and high-carbon steel. Each type varies significantly in properties.

Note that AISI carbon steels that named 10xx have a carbon content equal to .xx%. For instance 1006 has a carbon content of .06% and 1045 has a carbon content of 0.45%. Once a steel has carbon above .30% its weldability decreases below a threshold, but its hardenability increases above the threshold.

Low Carbon Steel

Medium Carbon Steel

High Carbon Steel

Alloy Steel

Alloy steel was created in order to further improve the properties of steel by combining iron and carbon with other alloys.

Similarly to Carbon Steels, AISI Chromoly Alloy steels named 41xx have a carbon content equal to .xx%. For instance 4140 has .40% carbon content.

Alloy Elements

Stainless Steel

Chromoly Steel

Tool Steel

Spring Steel

Weathering Steel

ASR is a mechanical and aerospace engineering firm that specializes in engineering design and analysis. If you are in need of engineering services then contact us today to speak to one of our experienced engineers for a free quote on your project!

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