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Motorsport Fabrication Fundamentals: Chromoly

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00:00 - The most commonly used high carbon steel in the motorsport fabrication industry is chromoly which is a form of steel containing chromium and molybdenum hence the name.
00:10 Alloying steel with chromium and molybdenum results in improved strength, corrosion resistance and hardness and if we unpackaged those elements and look at them individually, we can understand why chromoly has the characteristics that it does.
00:24 Chromium helps to increase the material's hardness and resistance to corrosion while molybdenum is present in other alloyed steels like stainless steel, tool steel and high temperature super alloys.
00:35 It gives the material greater corrosion resistance and improves weldability.
00:38 4130 chromoly is the typical grade that you'll most often see used in motorsport fabrication so it's useful to know what those numbers actually mean.
00:47 You'll remember from the last module that mild steel had the SAE grade number 1020 and those last two numbers told us that it had 0.2% carbon.
00:57 That makes it easly to understand that the SAE grade number 4130 for chromoly means that this tube has 0.3% carbon.
01:05 Now that we know what the carbon amount is, we still have the first two numbers to consider, in this case, 41 let's us know that this is a steel alloy that contains both chromium and molybdenum.
01:16 While these two elements both offer good corrosion resistance, it isn't high enough to completely resist corrosion in chromoly steel, though it's adequate enough to build components and leave them in their raw state.
01:28 You may have seen this before in off road racing where the complete chromoly tube frame structure is left unpainted, something that wouldn't be possible without the added chromium and molybdenum resisting corrosion.
01:40 So far we've been discussing 4130 chromoly which is likely to be the most common grade you'll want to use but this material is available in many different grades ranging from 4118 to 4161 with varying amounts of carbon in each.
01:54 4150 chromoly for example is a steel commonly used by the military for aircraft parts and rifle components.
02:01 Chromoly is available in solid bar stock, sheet metal, square tube, streamlined tube and even flat plate which is useful for laser cutting items.
02:11 In many forms of motorsport, using chromoly for chassis and roll cage structures is the industry standard and you'll find it's also used in many other parts like suspension, steering, and engine components.
02:22 It's worth noting that the alloys that give chromoly its high tensile strength also remove its maleability and make welding, grinding and bending more difficult.
02:32 What this means for us as fabricators is that we need to pay special attention to the construction of chromoly components.
02:39 One big consideration here is that a reaction occurs when heat is added to it and many chromoly components are heat treated after machining which is the process of heating the steel to a specific and tightly controlled temperature in order to alter the physical and chemical properties for the purposes of hardening and strengthening.
02:58 While this is an advantageous process, commonly applied to many chromoly components like flywheels and crankshafts, it can also lead to issues for the fabricator when it comes to welding.
03:09 Let's take a chromoly roll cage for example. the tube in its natural form is taken off the rack, measured, cut, bent, notched and positioned ready for welding.
03:17 After the weld is performed and the roll cage begins to take shape, we're left with a structure that has very uneven amounts of strength.
03:24 The welded sections are heat treated and are therefore physically altered compared to that of the unwelded areas.
03:30 In a perfect world, this roll cage would then be removed from the car once complete and annealed which is the process of uniformly reheating the roll cage to a temperature above chromoly's recrystallisation temperature, then cooling it down at a slow rate that reforms the structure of the steel to its original crystalline state.
03:49 Once annealed, the roll cage will be completely uniform and in the event of a large impact, would bend and soak up the forces evenly.
03:56 Obviously, having our roll cage annealed is for the most part impossible and without performing this process, we can end up with an extremely strong and brittle area around the welds, while the unwelded sections of tube are more malleable.
04:09 Unfortunately this can cause chromoly components to break around the heat affected area next to the weld which has been known to create serious injury and even death to occupants as a result.
04:21 If you're starting think that 4130 sounds like a terrible material to build motorsport components from, then that is definitely not the case but we do need to understand how to reduce the risks.
04:31 This primarily comes down to reducing the amount of heat that's placed into the weld and there are a few ways of going about this.
04:38 First of all, chromoly requires a little more time and attention when it comes to fit up between parts.
04:44 Provided the fabricator has done a neat job of the fit up, welding the relatively thin wall tube used for roll cages won't require too much heat.
04:52 Large gaps on the other hand will require a lot more heat input which will then be detrimental to the strength of the finished part.
04:59 The type of welding also comes into play here and this is why TIG is preferred since it places less heat into the completed weld and gives us a lot more control over the heat application in comparison to MIG.
05:11 Of course the welding technique of the operator can also be a factor when it comes to the amount of heat placed into the weld.
05:19 To further mitigate the risks in professional motorsport, we often see chromoly components produced with the help of computer aided design, followed by a validation process where the design is run through a process called finite elements analysis or FEA for short.
05:33 This ensures the part can support the expected loads it'll see in the event of a serious incident.
05:39 It's also important to consider the stresses that flexing places on chromoly components.
05:45 Unlike 1020 mild steel tube we discussed in the previous module, chromoly doesn't offer the same flexibility and with the added brittleness around the weld areas, flex can cause cracking and failure to a component if it's overstressed.
05:58 There are ways for us to overcome this though.
06:01 For instance, well thought out use of gussets will add enough strength to the heat affected weld area and allow the tube to flex in the unaffected areas, spanning the distance between joints.
06:11 Chromoly will always be the chosen steel for top level motorsport and once you understand the material, and you've perfected your technique on more forgiving materials like mild steel you can safely save a lot of weight and increase rigidity with the use of this material.

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