Practical TIG Welding: Chromoly
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Chromoly
05.53
| 00:00 | - Chromoly steel uses a mix of elements that make it favourable for motorsport applications where weight and strength are critical. |
| 00:07 | The material itself weighs the same as mild steel but its increased strength allows us to build components lighter by reducing the wall thickness of the material. |
| 00:16 | Chromoly is commonly used for complete chassis construction as well as suspension components and quite often these components are fabricated from very thin wall tubing. |
| 00:25 | This reduction in thickness along with a higher carbon content can lead to the material being very heat sensitive however and as a result the welding process requires a few very specific considerations for safe and strong welds. |
| 00:41 | The problem with chromoly isn't so much the weld itself but rather the heat affected zone either side of the weld. |
| 00:47 | If we input too much heat into the material while welding, it'll harden as it cools. |
| 00:51 | The end result in a brittle area on either side of the weld that's a potential failure point where cracking is likely to occur. |
| 00:59 | A strong chromoly weld starts well before picking up the torch. |
| 01:04 | Good material fit up is absolutely critical here as this is going to reduce the amount of heat needed to perform a successful weld. |
| 01:12 | This is because poor fitup results in gaps which then require more heat in order to fill or bridge these gaps between the two components with our filler rod. |
| 01:21 | We're ideally looking for a perfect fit up with no visible gaps at all, and the maximum gap that's acceptable is no more than a quarter of a millimetre but less is always preferable. |
| 01:34 | Once the ideal fit up is achieved, we also need to properly repair the material for welding to ensure that there are no contaminants or inclusions in the finished weld. |
| 01:42 | This can be achieved by using scotch brite or sandpaper to remove the darkened external layer from the chromoly, about 25-50 mm back from the weld area. |
| 01:51 | Lastly we need to wipe down our weld joint and filler rod with acetone to remove any oils or contaminants. |
| 01:59 | One misconception with chromoly is that we must use a matching chromoly filler rod to weld this material. |
| 02:06 | This would be true if we were post heat treating the part and normalising the entire structure but 99% of us won't be doing that. |
| 02:13 | The strength of the weld isn't an issue with chromoly and good technique coupled with ER70S-2 filler rod will produce a strong weld that has enough ductility to offer a little flex which results in a weld much stronger than the heat affected material either side of it. |
| 02:30 | Another common misconception is that chromoly must be pre heated or pre and post heated when welding it. |
| 02:37 | While this may be true for gas welding, it's not usually required for modern day TIG welding, at least on thin wall tube. |
| 02:44 | Our TIG process allows us to concentrate the heat into the weld join and minimise the effect on the material itself. |
| 02:50 | We can learn from this method though and reduce the thermal shock by implementing a slower rise to molten weld temperature and steady drop to recrystallization through our up slope and down slope settings. |
| 03:03 | The only caveat here is if you life in a very cold part of the world where your workshop temperatures are close to freezing. |
| 03:09 | Then pre heating prior to TIG welding can still be advisable. |
| 03:13 | Even in these climates, most of us would have some form of workshop heating for our own comfort which has the knock on effect of bringing the temperature of the chromoly into a workable range so this is still unlikely to be an issue. |
| 03:27 | Pre heating in this instance is aimed at raising the material temperature up to around 30 to 50°C. |
| 03:33 | Likewise for thicker sections of chromoly, pre heating to 200 to 250°C is advisable. |
| 03:40 | The pre heating process, when required, is normally achieved with an oxy acetylene gas torch. |
| 03:46 | Welding chromoly requires the same amperage and techniques as mild steel and the key to welding chromoly is keeping your arc length as small as possible to reduce the heat affected zone and using enough amperage to keep a consistent weld speed is paramount. |
| 04:02 | Pulse control can also assist in welding thinner materials and reducing the heat input while allowing us to keep our travel speed up. |
| 04:10 | A good chomoly weld should have a shiny appearance with some blue colouring and a uniform bead structure. |
| 04:16 | If the weld looks dull and grey in colour, this can indicate poor or insufficient shielding gas coverage. |
| 04:23 | Another potential cause is the torch being held too far from the workpiece, thus creating excessive arc length which increases the heat input into the part. |
| 04:32 | If you're finding imperfections in your weld like pin holes or craters, also known as porosity, or a dusty brown colouring at the sides of your weld, this shows that contaminants are getting into the weld pool. |
| 04:45 | This could be caused by a lack of shielding gas, a contaminated tungsten or a contaminant on the material itself. |
| 04:53 | If this is happening, the weld needs to be ground out, cleaned and rewelded. |
| 04:57 | You can't just weld over the top unfortunately as the impurity will still be there and continue to cause problems. |
| 05:05 | Another way to identify the quality of our chromoly weld and this applies to most ferrous metals but is quite important in chromoly, is keeping an eye on our heat affected zone. |
| 05:16 | As previously mentioned with chromoly, applying too much heat can lead to failures around the weld area due to the heat sensitivity. |
| 05:24 | If we start to see an excessive heat affected zone around the weld, it's a clear indication we need to put less heat into the part. |
| 05:31 | Some discolouration is to be expected but for maximum strength, we want to keep this to a minimum. |
| 05:38 | Aside from our actual welding technique, we can also reduce the heat input to the material by welding opposite sides of the part or smaller areas, allowing time for the part to cool between each weld. |
