Steel is one of the most reliable commodities on earth, with its unquestionable durability and strength. Steel structures provide long-term, stable performance. Steel does not expand or contract with moisture exposure, nor does it warp, split, crack or creep.
Steel is typically priced by hundredweight (CWT) — the price per 100 pounds of material. In some cases (e.g., materials market reports), carbon steel price may be reflected per pound.
For example, a CWT price of $50 equates to $0.50 per pound:
$50 / 100 pounds = $0.50
Additionally, units are often quoted in tons (1 ton = 2,000 pounds).
$50 CTW x 1 ton = 50 x 2000 lbs / 100 = $1000
When considering metal for a specific project, tensile strength and yield strength are two of the most important factors – especially for structural applications. Tensile tests determine tensile strength, yield strength, and elongation of a material. These properties are contingent on the alloy and temper as well as the shape of the material being tested.
Knowledge of the yield and tensile strength is significant because each impacts the production and use of steel (and various other materials, but we will focus on just steel here).
So, what’s the difference between yield strength and tensile strength? The most significant difference is that tensile measures a catastrophic failure, where yield just measures the start of permanent deformation.
Below we go into more details about both of these and discuss elongation with respect to tensile strength.
What is yield strength?
Yield strength is the maximum stress that a material can withstand before changing shape permanently. It is an approximation of the steel’s elasticity. If stress is applied to the metal but not enough to reach its yield point, it will return to its original shape when the stress is removed. When a material is stressed by an amount less than the materials yield stress, it will only undergo elastic (reversible) strain, and no permanent deformation will occur.
When stress exceeds the yield point, the steel will not bounce back. Thus, yield strength illustrates the upper limit of the stress that can be safely applied to the metal, making it imperative to know when designing structures and components.
What is tensile strength?
Tensile strength is the maximum stress a material can sustain before fracturing — the resistance of steel until it reaches its breaking point. Once a piece of steel is stretched past its tensile or breaking point, it breaks.
One of the most common ways to measure the tensile strength of an object is the tensile test (a.k.a. the tension test). During this procedure, the test specimen (usually a cylinder shape) is loaded into a device that clamps it at one end and applies an axial force on the other. The machine gradually and continuously elongates the specimen at a standard rate until failure.
What is elongation?
Elongation is the percentage of stretch from the steel’s original length to the point of failure, showing how ductile the steel is. Ductility is the ability of a material to be elongated in tension. The higher the ductility, the more malleable the product is. Elongation is a useful measurement for determining if you’re choosing the right product for a project.
Ductile failure is the initial stage of failure, where it is pushed beyond the yield point to permanent deformation. It can technically still be in one piece, but the metal is critically and permanently compromised.
Brittle failure is the final stage where the tensile strength measurement is taken. A brittle failure is when a metal snaps or cracks, often with no evidence of ductile failure.
What is coating weight, and how is it measured?
Galvanizing is a widely used method of applying a protective zinc coating to steel or iron to prevent rusting and corrosion. Coating weight is the amount of zinc coating applied to a product for a given surface area. ASTM (American Society for Testing and Materials) A123, A153, and A767 specifications govern the zinc coating standards for material classes during the hot-dip galvanizing process. The amount of coating is specified by either thickness or weight per surface area. The specifications include tables providing specific requirements for thickness or weight per surface area based upon the steel part type and the measured steel thickness.
There are two methods utilized to measure the coating weight of galvanized steel.
This method measures the weight of a steel component after it has been cleaned, then again after galvanizing is completed. It is only applicable for single specimen samples.
Every coated steel sheet product has its own coating weight designation system defined in the appropriate ASTM standard. A653/A653M is the most widely used ASTM metallic-coated sheet standard, covering hot-dip galvanized products.
One coating weight designation system in this standard uses descriptors (e.g., G60, G90, etc.). The “G” means the coating is galvanized (zinc), and the numbers refer to the weight of zinc on the surface of the steel sheet in inch-pound (English/Imperial) units.
Using G90 as an example:
- The total coating weight on one square foot of sheet (i.e., both sides of the sheet) has a triple spot test (TST) average minimum of 0.90 ounces.
- Each square foot of surface would have a minimum of 0.45 ounces if equally distributed to both sides of the sheet.
The second technique, weigh-strip-weigh (again, only appropriate for single specimen samples), measures the weight immediately after a galvanized part is cooled, then again after the coating has been stripped off the piece with an acid solution. There are four steps for testing with this method:
- Run the steel panel through the iron phosphate pretreatment
- Weigh the panel to determine the baseline weight
- Dip the panel into a chromic acid bath to remove the phosphate coating from the panel
- Weigh the panel again and subtract that weight from the baseline weight to find the phosphate coating weight
If the weight is within the acceptable range, the chemical balance of your iron phosphate bath is within spec and is ready to use.
What is the Rockwell Scale?
The Rockwell scale is the most widely used hardness test method and is typically easier to perform and more accurate than other hardness testing methods. It involves the application of a minor load followed by a major load and then noting the hardness value. Its primary advantage is its ability to display hardness values directly, thus eliminating tedious calculations involved in other hardness measurement techniques.
Various indenters may be used, ranging from a conical diamond with a round tip for harder metals to ball indenters with a diameter ranging from 1/16″ to ½” for softer materials.
There are several different Rockwell scales; each is used for different materials. The most commonly used are the “B” and “C” scales.
The B-scale is used for softer materials (such as aluminum, copper, and softer steels). It employs a hardened steel ball as the indenter and a 100kg weight to obtain a value expressed as “HRB.”
The C-scale, for harder materials, uses a diamond cone (Brale indenter) and a 150kg weight to obtain a value expressed as “HRC.”
The penetration depth is converted to a scale in which a harder material gives a higher number. The Rockwell hardness number is a combination of the measured numerical hardness value followed by the letters HR and the scale letter. For example, a hardness value of 70 on the Rockwell B scale is reported as 70 HRB.
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