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When TIG weld cracks, the deformity can jeopardize the integrity of the weld and cause delays, downtime, and severe frustration. Thankfully, as with any part of the welding process, you can not only understand why your TIG welds crack, but you can also learn solutions to help your welds to stop cracking.
What are the reasons why your TIG welds crack, and how can you prevent them?
Reasons why your TIG welds crack:
- The temperature is too hot while welding and is allowed to cool too quickly.
- Your base material is a steel that is prone to residual stress and rapid cooling.
- Hydrogen enters the weld through the atmosphere, filler metals, shielding gas or mill scale on your base material.
How to Prevent them from cracking:
- Reduce your heat input and don’t allow the weld to cool too quickly by using a backfilling technique to create thicker welds.
- Pre-heat the base material before welding.
- Perform a post-weld heat treatment to drive diffusible hydrogen from the weld and relieve material stress.
The main types of TIG weld cracks that occur fall under the two main categories of hot cracking and cold cracking.
Hot cracking takes place at temperatures over 1,000 degrees Fahrenheit and occurs almost immediately when the weld solidifies. It is the cause of almost all cracking in aluminum weldments. This cracking mechanism is also called hot shortness, hot fissuring, solidification cracking, and liquation cracking.
There are three types of hot cracking – segregation, bead shape, and crater.
This is a type of centerline cracking, which usually comes from the middle of the material being welded. It can happen when you are welding metals that have higher sulfur and phosphorous content, or ones with zinc plating or galvanized coating. It also occurs on materials that have been painted or primed.
There are several options when it comes to trying to prevent this type of cracking:
- Use steels with low residual phosphorus and sulfur (less than .05 percent).
- Use less heat, as excess heat increases the chance of cracks.
- Modify the joint design so the depth-to-width ratio is not too high or too small.
- A higher throat thickness or a lower concavity in the weld will cut down on cracks.
Bead Shape Cracking
A bead-shaped crack is usually due to joints that aren’t fitting correctly. This happens because the joint issues lead too much stress being put on the bead shapes, causing it to crack.
This is also the reason why a bead crack can happen due to a fit-up that does not facilitate contact. Finally, bead shape cracks can be created from excessive heat due to a voltage that is too high.
You can prevent bead shape cracking by:
- Having good part fit-up and joint design.
- Adhering to the recommended parameters for your application.
- Lowering the voltage as necessary.
- Avoiding wide-weave techniques as these create a concave bead. Try using a weld drag angle to create a more convex weld bead.
Crater cracking happens if you stop welding prior to completing a pass on a weld joint. A stopped weld like this can cause the outside of the crater to cool faster than the inner area, which can lead to cracks forming. These cracks can appear from any direction and there can be multiple.
To prevent crater cracks, use a backfilling technique to provide more exceptional thickness to the crater. This entails backing up slightly to fill in the area at the end of the weld prior to extinguishing the arc. Also, it would help if you were certain to fill in the spaces between tack welds and your weld pass, depositing thicker tacks and filling in the craters.
Cold cracking, which is also called hydrogen-induced or heat-affected zone (HAZ) cracking, occurs at temperatures below 600 degrees Fahrenheit. The cracking doesn’t appear until hours after the weld cools. In some cases, it might not appear for days.
Thick materials are most commonly affected by cold cracking due to their larger heat sink. This is due to cooling that happens too quickly, which puts too much pressure on the base material. The base materials that are more likely to be affected by cold cracking are those with high alloy content or high carbon materials. This is because they are not as pliable due to their durability.
There are a few ways to prevent cold cracking. Usually, they are based on making sure that there is as little excess stress on the base metal.
Making sure that the weld isn’t full of hydrogen is also essential, as hydrogen will weaken the weld. To prevent excess hydrogen from getting into the weld, clean the base materials of any rust or oils, make sure that the weld area is ventilated properly, and always use the correct amount of shieling gas.
Another way to prevent cold cracking is called back-stepping. Back-stepping can relieve excess stress that your weld may be under. There are a few steps that you have to take to back step your weld.
To back-step, weld a short section near the edge of the material, until reaching the edge. Then, weld a part directly behind the previous one, stopping once you’ve overlapped with the last section a bit. Repeat this until getting to the other end of the weld.
Another preventative measure against this issue is a quick pre-heat of the base metal before starting the weld. It can slow the cooling rate and help maintain the ductility of the weld and base material. Post-weld heat treating (PWHT) helps, as well. PWHT drives the diffusible hydrogen out of the weldment and relieves material stress.
More preventative measures you can take:
- Keep filler in a similar temperature to your welding area, or let the material settle into the temperature for a few hours before starting the weld.
- Use a low-hydrogen filler, like H4 metals.
- Make sure that the base material is clean and dry! This has previously been stated because it is crucial.
- Keep filler in dry environments and in the packaging that you bought them in until directly before welding.
Other Types of TIG Weld Cracking
While the type of cracks mentioned above is the most common and most devastating, there are a few more that you might need to know about. These cracks are usually much smaller, and some can even be fixed if noticed quickly enough.
Arc Strike Cracking
This occurs when the arc is struck but the spot is not welded. This happens because the spot is heated above the material’s upper critical temperature and then cooled very quickly.
This can lead to the formation of a steel crystalline material called martensite. Martensite can cause micro-cracks in the weld, so if the arc is struck in this way, further cracking should be stopped by welding over it immediately.
Hat cracks are a crack that creates a flared-out shape, like the bill of a hat, in the middle of the weld. They usually move through the weld from the line of fusion and are commonly caused by too much voltage or not enough speed during the initial weld.
Reheat cracks show up with metals that do not have proper creep ductility, which is to say that they are easily breakable under pressure. When these more brittle metals are treated with heat, these cracks can occur once the material begins to cool. This usually happens to low-alloy steels like chromium.
Root cracks commonly happen when a mistake is made near the start of the weld. This can be using the wrong filler for the job or an electron current that is too low.
Toe cracking happens if too much moisture comes into contact with the weld. This can be via the atmosphere or within the welding area. It is a surface crack that is easy to find at the toe of the weld.
A longitudinal crack is going to be a crack that appears length-wise through the weld. Three types of this crack exist – check cracks, root cracks, and full centerline cracks.
A transverse crack is going to appear across the width of the weld. They are usually due to a shrinkage in the weld length-wise. This shrinkage can be attributed to the metal cooling too quickly or the use of the incorrect base material.
TIG weld cracks can completely ruin a weld, leaving it weak and unsightly. This can be frustrating, especially after going through the lengthy process of completing the weld. Unfortunately, cracks are very common. This is especially true in the case of new welders who don’t understand the process as deeply.
While there are many things that you can do about cracks after the fact, preventative measures can be taken. In understanding the process of TIG welding, you should be able to more intuitively prevent mishaps like cracking in the future. Hopefully, this article was able to give you some of that insight.