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Motorsport Fabrication Fundamentals: Mild Steel

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Mild Steel


00:00 - By far the most used material in OEM vehicle construction is mild steel, which is available in a wide variety of shapes and sizes, including round and square tube, sheet, plate, bar stock and more.
00:12 The mild in mild steel refers to its low carbon content.
00:17 The actual amount of carbon in steel is only a very small percentage, with the highest steels only containing around 2.1% carbon and mild steel only contains between 0.5 and 0.25% carbon.
00:31 This means that it's quite soft and malleable and although that sounds like it might be a drawback, it's not necessarily a bad thing since being malleable means that this steel has a high resistance to braking or shattering under stress.
00:43 It's important to understand that even though mild steel is known as a low strength steel, its properties can still be very useful in motorsport applications where loads or stresses are relatively low.
00:55 It's also a low cost material that's widely available and is easy for us to work with.
01:00 Welding mild steel is also a great gateway into motorsport fabrication since low carbon steels are easy to weld and they don't harden from heat which happens with the higher carbon steels like chromoly.
01:12 This offers us a lot of freedom when welding because we don't have to be so concerned about overheating and adversely affecting the surrounding area.
01:19 A MIG welder inputs more heat into the weld area in comparison to TIG welding so the MIG is an ideal option for welding mild steel.
01:28 As discussed in the MIG welding option, MIG welding higher carbon steels like chromoly isn't recommended as the increased heat can adversely affect the surrounding area, making the steel brittle, and this can cause cracking and breakage.
01:42 Mild steel also offers easy machinability with its low carbon content and being soft and malleable steel, it offers our drills, hole saws, cutters and grinders very little resistance.
01:54 Again, this material won't harden with heat and that gives us a little bit more room for error if our drill is a little blunt or our speeds or feeds aren't exactly as they should be.
02:04 We've been using the term low strength steel in this module so it's worth having a basic understanding of what strength is and how it's measured.
02:12 There are two main metrics used in motorsport, tensile strength and yield strength.
02:18 Tensile strength is the resistance that the steel has to failure or breaking due to a tensile force, or in other words, if a force is applied in such a way as to attempt to pull the material apart.
02:29 Yield strength on the other hand is the maximum stress that can be applied to the material before it will be permanently deformed or in other words, it won't return to its original shape when the stress is removed.
02:39 Tensile and yield strength are both measured in MPA which stands for mega pascals.
02:45 Specialist motorsport material suppliers will offer mild steel tube that easily surpasses the strength needed to conform to our motorsport governing body's regulations, which for the most part really only applies to rollover protection structures.
02:59 For example most mild steel tubing regulations stipulate that our tube has a minimum yield strength of 350 mega pascals before deformation.
03:08 So how do we know that our material meets these standards? It's actually fairly simple as all steel produced will have a code that defines the steel and makes it quickly recognisable to the fabricator.
03:19 Let's look at an example here.
03:21 This mild steel tube has the code CDS1020, so let's take a moment to work out what that actually means.
03:28 CDS stands for cold draw seamless and that means the process of making this tube involves forcing a solid bar onto a pierced mandrill to form the inner diameter and an outside die to form the outside diameter.
03:41 The benefit of this is that we end up with a uniform micro structure, tight tolerances, high strength to weight ratio, high tensile strength, a thinner tube wall and a superior surface finish when compared to other methods of forming tube.
03:56 The numbering system for coding steel makes each number indicative of its class.
04:01 The 1 in 1020 means that this is a carbon steel and the 0 indicates that the steel doesn't have any major secondary elements present such as high levels of sulfur that improve machinability.
04:13 The third and fourth numbers indicate the level of carbon present in the steel.
04:17 In this case 20 means that the material contains 0.20% carbon.
04:23 As good as mild steel is, it does have some downsides, specifically around its weight and corrosion resistance.
04:30 Mild steel components will always be heavier than their higher carbon counterparts because the stronger high carbon steels allow us to use thinner material to achieve the same amount of strength.
04:41 In terms of tube structures, the wall thickness can almost be halved by using higher carbon steel such as chromoly.
04:48 Due to the material's higher iron content, mild steel also rusts very quickly.
04:53 This corrosion will continue to attack the surface if not sealed or oiled and if left unchecked, rust will develop pit marks and eventually flake away, compromising the strength of the steel.

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