Top Tips for Proper Weld Preparation
Everyone knows just how important welding preparation is – and preparation details matter. Using the wrong disc or wire type for a welded material or applying an unsuitable cutting method can cause bad weld defects. The weld preparation tips below will help you avoid weld discontinuities and defects and save you from costly and time-consuming rework.
Know the Material You Plan to Weld
Some metals are soft, others very hard. Some tolerate more contaminants, others not even a little. Welding plain mild steel is the easiest. It's not too soft or too hard and can take more contamination than other materials like aluminium or titanium.
Material hardness determines the abrasive type and hardness you need to use. You wouldn't want to use a wire wheel to clean aluminium because you could embed the impurities deep into the metal. Likewise, you'd need a very hard abrasive to clean nickel alloys or cast irons.
The hardness of the materials can affect the end result in less obvious ways too. For example, stainless steel is harder than mild steel. So, when you grind it, the resistance is higher, causing it to heat up. However, stainless steel doesn't tolerate high heat well, which could impact the weld quality. So, you'd need an abrasive with a heat-dissipating coating to help the heat flow away from the stainless steel. For example, the Kronenflex A24N Supra is an excellent choice for grinding this material.
All metals have surface contaminants that must be removed prior to welding. Steels can have mill scale, galvanised steel has a zinc layer, while non-ferrous materials have surface oxides. Aluminium, magnesium, and titanium have surface oxides that must be removed. But, these metals oxidise quickly. So, you need to weld fast after the pure metal is exposed to the atmosphere. If you clean the joint but don't weld it soon after, you might need to clean it again before welding.
Soft metals, like aluminium, can cause wheel loading when using standard grinding wheels. The material sticks to the wheel, causing it to lose effectiveness. It's recommended to use grinding wheels specifically designed for aluminium. They can prevent wheel loading and ensure a high material removal rate.
Pre-Cleaning Requirements for Different Welding Processes
The STICK welding process tolerates surface impurities better than other arc welding processes, but it requires extensive slag removal after each weld pass.
MIG tolerates slightly more surface contaminants than TIG, while TIG is the most sensitive process of them all. However, TIG produces the cleanest welds and requires almost no post-weld cleaning, while MIG can require silicon removal and surface finishing. You can read more in our MIG vs. TIG comparison.
The STICK welding process is the best choice when welding carbon steel that has rust on its surface and other impurities, like paint. Of course, you should remove as much of the surface contaminants as you can, but you'll have a better chance of achieving a sound weld compared to TIG.
Start with Cutting and Bevelling
Unless the material is very contaminated with oil, grease, dirt, paint, and rust, you should start with cutting and bevelling first. Generally, cleaning the joint comes after the joint is cut and bevelled.
Sheet metal can be cut with an angle grinder and a cutting wheel. Likewise, this is an excellent way to quickly cut thin-walled pipes and various profiles made from carbon steel, stainless steel, and non-ferrous materials.
For higher precision and better productivity, it's recommended to use a plasma cutter, like the Hypertherm Powermax 65 SYNC. Plasma cutting works wonders for sheet metal and very thick sections alike. The Powermax 65 SYNC can cut up to 20mm thick at the rate of 500mm per minute with a fine-cut finish, requiring little to no post-cutting touch-up with a grinder. Plasma cuts all metals, making it the most versatile of all high-productivity cutting tools.
You can also use a more traditional method of metal cutting — the oxy-fuel. A quality torch, like the Harris LPG Cutting Torch, can make clean cuts in the hands of a skilled operator. However, oxy-fuel cutting can't be used for stainless steel and non-ferrous materials. It also poses a higher safety hazard due to the use of flammable gases. The cost of fuel, storage, and transportation must be factored in when evaluating the cost-effectiveness. However, oxy-fuel is still widely used for cutting and bevelling steel due to its ease of use and wide historical adoption.
Once you cut the material to meet the size requirements, it's time to bevel the joint's edges. The most common weld joints are the butt, lap, tee, edge, and corner joints. They are often bevelled into V, single, or double bevel configurations to ensure sound weld penetration when welding thick sections.
Speaking of bevelling, you can also use a plasma cutter, but it takes more skill than just slicing the metal with the plasma torch. Here, oxy-fuel beats plasma in the ease of application.
However, the two most straightforward ways to bevel the metal are using a powerful angle grinder with a suitable grinding wheel and using an electric bevelling machine. Electric bevelers are often used to bevel pipe joints to achieve the high level of precision required for weld accuracy, fit-up, distortion control, and preventing burn-through.
Precision and uniformity can make or break the cutting and bevelling process of weld preparation. It's extremely important to choose tools suitable for the job and to your operator's skill level. Inconsistent weld preparation can lead to weld discontinuities and defects, leading to costly rework. For example, grinding away too much material in a single spot of a long single bevel joint can cause melt-through in that area with less material. So, choosing the right tool goes a long way in preparing the weld for success.
Clean Before Welding
The freshly cut and bevelled joint surface is already cleaned, but you'll also need to clean the surface within an inch from the joint on both sides. You must remove the surface oxides, mill scale, paints, rust, oils, and grease.
First, start by cleaning the metal oils with acetone. Next, use a wire brush or an angle grinder to remove surface oxides and other impurities that are difficult to remove. You'll need a dedicated wire brush or grinding wheels for stainless steel, aluminium, nickel alloys, and titanium. Cross-contamination with carbon steel can cause weld defects and should be avoided. You'll also need a stainless steel wire brush for all metals other than carbon steel. You can use a standard carbon steel brush for mild steel but not for metals like stainless steel and aluminium.
Flap discs are great for removing surface oxides, mill scale, and other hard impurities. They are easy to control and move along the joint line. For example, the Klingspor SMT 974 Special Ceramic Flap Discs are an excellent abrasive for high stock removal with an exceptionally long service life. There are many flap disc types you can use, depending on the material you are working with. For example, you'd want to use the Klingspor MST 924 Special Mini Ceramic Flap Discs for stainless steel.
Preheating May Be Necessary
Many welding codes require preheating and maintaining specific interpass temperatures, making preheating a vital preparation step. You can use open flame heating torches to reach the required temperatures or an induction heating system.
Unlike the manually applied open flame, induction heating provides a uniform heat gradient. Uniform heat application ensures consistent material response to welding. For example, welding a joint where parts of it are 50-100°C colder than the rest of the joint could cause reduced fusion, inconsistent distortion, and other defects. It's easy to overheat certain parts of the workpiece and underneath others when using open flame. That's just one of the reasons why induction heating is better than open flame. Induction systems don't rely on dangerous fuel and heat far faster than open flame heating.
We have one of the most advanced induction heating systems in the world ready for hire – the Miller ProHeat 35. Buying induction heating gear is not always cost-effective, but hiring it provides the ROI immediately. It is often much easier to hire an induction heating power source and accessories than to arrange oxy-fuel transport and meet safety codes for safely handling pressurised flammable gas cylinders. But, most importantly, induction heating saves you time and resources while providing significantly better thermal properties.
Induction heating is particularly beneficial for welding pipes, pressure vessels, structural steel, and other applications where strict adherence to welding codes is required. Using induction heating makes it easier to pass NDT, meet WPS specifications, and have documented logs of workpiece temperatures in various stages of production.
Need Additional Help? Alphaweld is Here for You
If you need clarification on the required abrasives or cutting methods most applicable to the material you are working with, contact the experts at Alphaweld. Call (08) 9456 8000 or send us an online enquiry and we'll be happy to help.