These topics are on the agenda of Steel Warehouse's Technical Group.
Other topics (stay tuned for more information):
Green Steel
A reduced carbon footprint is becoming a high priority action item.
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The technical topics discuss bending and weldability. For further explanation, see below.
Bending and Bend Guarantees
This section will address free bending using a die, but there are other types of bending, too, such as roll bending. There are four basic steps to follow when bending a piece of steel:
1. Determine the rolling direction
2. Clean up the edges
3. Use the appropriate bend radius and die opening for the steel grade
3. Adjust for springback
1. Determine the rolling direction
Steel that has been produced at a hot strip mill has elongated, anisotropic grains, meaning it's properties going "with" the grains are different than it's properties going "against" the grains.
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A bend going "with" the grains (in red) is a longitudinal bend. A longitudinal bend is also called a "bad way bend" because the material is not as strong in this direction.
A bend going "against" the grains (in green) is a transverse bend. Transverse bends are better for forming because the increased amount of intersecting grain boundaries provides strength.
2. Clean up the edges
Discontinuities serve as origins for crack propagation. Grinding away burrs, laser dross, and any other blemishes is important for a successful bend.
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3. Use the appropriate bend radius and die opening for the steel grade
Bend radii are a function of material thickness, and are usually given as a "xt" number - 1t, 1.5t, 3t, etc. This means that the guaranteed minimum bend radius for a particular grade is its thickness multiplied by the bending factor. Transverse and longitudinal bends usually have different bending guarantees, with the transverse bend being tighter.
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The length of the part being bent, the size of the punch radius, and the die opening also play a role in a successful bend. For more information, please contact us.
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4. Adjust for springback
After bending, high strength steels will springback, meaning they will lose some of the bend - up to 20 degrees in very high strength steels. To achieve a particular bend radius, the steel must be sufficiently over-bent.
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Weldability
The biggest factor in whether a material is weldable is the composition of the base metal and filler metal. If there is a lack of metallurgical solubility, brittle compounds may be formed during joining.
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When welding steels, the presence of hydrogen, residual stresses, and the formation of martensite can lead to cracking.
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Many applications have a "carbon equivalent" requirement to limit the amount of martensite likely to form during welding. The carbon equivalent, or CE, is a number generated from an equation that relates alloying elements to a percentage of carbon.
In steelmaking, the properties of the steel are determined by many inputs. Cooling rate, carbon percentage, alloying elements, and many other variables affect the steel's characteristics. Metallurgical research allows us to predict the microstructure given the carbon content and temperature, but adding alloying elements can greatly alter the results.
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This is a problem for applications that require rapid heating and cooling, such as welding. If the steel is too hard, it is likely to crack after it's been welded. The steel can be heat treated before and after welding to prevent cracking, but how do you decide which steels needs to be heat treated if they are heavily alloyed and you cannot predict their characteristics?
Because we know how plain carbon steels are likely to react to welding, we needed a method to relate alloyed steel to plain carbon steel.
Enter the CE:
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The CE equation allows us predict the hardness of an alloyed steel based on its plain carbon steel equivalent. If the CE is 0.35% or below, the risk of cracking after welding is low. Above 0.35%, and especially above 0.40%, heat treating may be necessary.
More CE formulas exist, and many with increasingly detailed equations. As the result the range of steel products expands, more specific equations better account for the complexity of advanced steels.
