Weld Distortion

Stock Allowance Explained: Why Manufacturers Add Extra Material

Stock allowance is material that is intentionally left on parts and welded assemblies at various stages of production. A necessary insurance policy, stock allowance enables manufacturers to compensate for distortion or warping by machining fits at intermediate stages of fabrication and at final machining. Since the mechanics of machining and welding are universal, every manufacturer of metal fabrications must consider stock allowance both during planning and intermittently as the project progresses.

Quantifying Stock Allowance

The appropriate level of stock allowance is almost always determined on a case-by-case basis. Planners must analyze the entire project from start to finish before any metal is cut, but stock allowance also depends on a series of small decisions made over the course of the project. Accurately assessing this allowance requires an understanding of how each set of parts will go together, and how each affects the next assembly.

Considerations start at the raw material stage as planners work to answer the following questions:

  • What will it cost to pay for the extra material up front?
  • Do we need to buy thicker material in order to have this extra stock, or is it just a matter of cutting the rough part a little bigger?
  • How much extra stock is enough (too little is as bad as none)?
  • What will it cost to remove the extra stock?
  • Is it a separate machining setup and operation, or just a few more cuts in a setup that's already taking place?
  • Do you need the extra stock on thickness or just on the outline?
  • Can you start with standard material sizes?
  • How is the part or assembly going to react to cutting, machining and welding?
  • Will it distort out of tolerance or is there enough tolerance to allow some movement?
  • Do we need to machine the part for a mating assembly — and where does that material come from?
  • Finally, how do we recover if the part or assembly goes out of tolerance and we don’t have stock to fix it?

The Dangers of Under-Budgeting Stock Allowance

Leaving too much extra material can be expensive, but failing to leave enough can be even more costly to both schedules and budgets.

In some cases where stock allowance is insufficient, manufacturers will have to add replacement material by welding, straightening the part or modifying the mating piece to accommodate the deviation. In the most severe cases, the part may not be usable at all, forcing the team to start all over again, which comes with obvious implications for both cost and schedule.

Stock allowance is a type of insurance that enables manufacturers to compensate for distortion and warping. Determining how much extra material to include is a delicate art form that comes with experience. To get it right, manufacturers have to plan up front, but they also must adjust and revisit their calculations as the project moves forward.

Weld Distortion

Weld Distortion in Fabrications: Anticipating, Planning and Reacting

Weld Distortion: The Basics

It’s a basic fact of metal fabricator’s lives that all welds shrink as the molten metal cools. This weld shrinkage introduces stresses into the fabrication and results in local distortions that affect the final shape of the part.

Welds shrink in all directions. Usually, the effect is greater in one direction than another. Since the parent metal away from the weld joint does not shrink, the uneven stress distribution causes the joined parts to distort.

This weld induced distortion is not easy to predict. This is especially so in more complex structures where multiple parts are welded together, with each weld adding its own distortion to the whole.

Some simple cases illustrate the basic distortions.

If two flat plates are welded side by side, the finished assembly will assume a shallow vee cross section and it will be slightly curled over its length. The vee is sharper if all the welding is all from one side.

If those two flat plates are butted together and welded to make an “L” or “T” right angle cross section, the angle between the legs will typically be less than 90° on the welded sides because the weld cross section shrinks.

If the flat plates are formed into cylinders and welded, the distortions are different. The longitudinal welds will flatten the curve of the formed shape at the weld, producing a reduction in the local radius at the seam. Typically, the cylinders stay straight because the formed cylinders are relatively stiff along their length. If two cylinders are then butted together and welded, the circumferential weld will shrink inward, producing a locally smaller diameter at the weld.

The degree of each of these distortions is influenced by the configuration of the weld joint, by the size of the weld along with the relative stiffness of the joined sections and even by the heat input of the chosen weld process.

It is possible to predict the weld shrinkage and the resulting distortions for simple cases like this, and there are more sophisticated methods to predict distortions in more complex cases. However, it often comes down to experience to anticipate the degree of shrinkage, where distortions will occur and to what extent the distortions will affect the final part.

Once the potential shrinkages and distortions are identified, designers, manufacturing engineers and welders can mitigate the effects to produce a finished part that meets all the customer’s expectations.

Planning for and Mitigating Weld Shrinkage and Distortion

Mitigating the effects of weld shrinkage and distortion is a function of good design, sound planning and careful welding practices.

Designers can start the mitigation process by minimizing the size and number of welds and by selective placement of welds. Placing welds at thinner sections, for example, can minimize distortion. Specifying double sided welds where practical gives welders an opportunity to balance out the distortions as they proceed. In some cases, designers can specify the degree of welding required for strength without specifying a particular weld configuration. A typical callout that allows this freedom is “Complete Joint Penetration” (CJP). This allows the manufacturer to use their expertise to select a weld joint design that will minimize weld distortions. Another way is to allow fabricators to reduce the number of welds by combining and machining several parts from a single piece of metal.

Manufacturers have several techniques available to them to control, anticipate and minimize the effects of weld distortion.

Manufacturing engineers can specify the sequence of assembly welds to take advantage of the accumulation of stiffness as parts are added to the assembly. If the customer can allow some leeway in the weld configuration, manufacturing engineers can minimize distortions by selecting weld designs that produce a more balanced joint with less distortion. Manufacturing engineers may also be able to reduce the number of weld joints and orient them in a way that reduces the accumulation of distortion. An important consideration for manufacturing engineers is to leave extra material on weld joint interfaces between subassemblies so surfaces can be trued up, including an allowance for shrinkage, in a planned machining step. Similarly, extra material should be allowed on critical surfaces for machining to print after all the fabrication is complete.

Welders can reduce weld distortions by keeping weld deposit sizes within the limits of the appropriate welding standards and by sequencing their weld deposits minimize heat input and to balance out the distortions as they occur. Bracing is another technique that is used to good effect, especially if the assembly and welding can be sequenced to be self-bracing.

In a production environment, where multiples of a welded structure are made, fabricators can often afford to make practice pieces to iron out the fabrication process to minimize difficulties at the production level. Specialty fabricators don’t often have this luxury, so experience becomes an important factor in anticipating the degree that welds will shrink and the direction and extent of the resulting distortion. Using the techniques and practices described above, the effects of weld shrinkage and distortions can be minimized and accounted for in subsequent processing.

Ultimately, distortion in a weldment comes down to the accumulation of weld shrinkages. Planners, therefore, have to examine each weldment individually and decide how it is going to be built and welders need to follow best practices to minimize the distortions caused by each and every weld.

The complexity of the welded fabrication, the number of welds, their placement and geometry, and weld the weld processes and practices all have an impact on distortion levels. It is critical to work with manufacturers who understand weld distortion and have the experience and skills to predict it, plan for it and mitigate it.

Weld Distortion

Pre-Stressing vs Stress Relief: Dealing with Weld Distortion

Even minor stress can cause significant issues. It’s true in life – and in weld distortion. Stress can ruin a weld, slow down a project, or result in a costly error. Stress relief and pre-stressing are two techniques for mitigating unwanted distortion. Both methods — which are essentially mirrored opposites of each other — are designed to control the finished shape of parts.

Weld Distortion

The Importance of Working to Predict and Minimize Weld Distortion

Distortion can negatively impact any weld. It is critical to predict and minimize weld distortion to maintain predetermined dimensions. Manufacturers that fully understand weld distortion are better prepared to anticipate it and work to mitigate its negative effects.

What is Weld Distortion?

Weld distortion occurs when weld metal, or the adjoining base metal, contracts and expands. Several variables can result in weld distortion, including temperature variations that cause a change in the physical properties of the metal, or when the welding process is applied unevenly to one side of the weld or the other.

Every part has a strict dimensional requirement that could be put in jeopardy if the material distorts during welding. Manufacturers must minimize weld distortion to achieve the predetermined dimensions required for the design to function correctly.

There are three types of weld distortion:

  • Transverse: This type of distortion occurs across the weld joint.
  • Longitudinal: Distortion that occurs along the length of the joint.
  • Angular: This distortion creates an angle on a surface that was supposed to be flat, for instance.

All three forms of weld distortion occur to some degree or another at the same time in every real-world application. Manufacturers use different techniques to limit each one individually and to control the others.

Planning to Minimize Weld Distortion

Before the first arc is struck, thorough planning is required to predict and mitigate weld distortion. Some weld distortion can be minimized during the weld itself. Other weld distortion is unavoidable and must be corrected after the weld is complete.

The first step of planning is to classify each segment of the welded assembly (or weldment) into one of three different categories of tolerances:

  • Tight tolerance
  • Medium tolerance
  • Loose tolerance

The tight tolerance sections are dealt with first. Since some degree of weld distortion is inevitable on tight tolerance welds, the team must plan for correcting this unavoidable distortion post weld. This requires them to add the appropriate amount of extra material eto machine away after the weld is complete, making the finished surfaces meet their pre-planned dimensions.

Next, the team must plan for medium and low tolerance welds.

The decision of whether to mitigate distortion during the weld or to rectify distortion afterward is made on a case-by-case basis. If they decide to work to limit distortion during the weld, the team must:

  • Choose fixture designs that limits distortion.
  • Hold and clamp parts rigidly during welding.
  • Choose weld joint designs that produce less distortion — smaller welds create less distortion than bigger welds.
  • Design a sequence of assembly that can reduce distortion. For example, they may build some rigidity into the design of parts. They may also put the critical framework of a welded assembly in place early so it has its own structure. Then, they can add more components later to avoid unnecessarily large distortion.

Post-Planning: Minimizing Distortion During Welding

The last line of defence against distortion is the people doing the work. The manufacturer must hire, train and rely on welders to be experts in welding distortion. They must offer their teams support, such as prevention training, dimensional measuring equipment and inspection personnel with high-tech tools that can detect and limit distortion throughout the welding process.

It is critical to stop and assess at pre-planned breaks throughout the process.

The engineering and welding staffs should work together during these planned pauses to monitor distortion as they go, re-evaluate and adjust to limit future distortion. If the first set of welds is bending to the right, for example, they will recalculate and compensate to the left on the second set of welds.

These periods of stopping and assessing cannot be arbitrary — they must be built into the planning phase.

Dealing with Unplanned Distortion

If distortion occurs that wasn't predicted, teams generally use mechanical means to correct the problem. Straightening is a process that forces the part back into the correct shape to limit remaining distortion. In other cases, the team may apply significant mechanical force by putting the part in a massive press.

In other cases, teams literally fight fire with fire.

Sometimes the best remedy is to attack the distortion using the same mechanism that caused the problem in the first place — heat. A process called flame straightening requires teams to use a torch or add some weld in a different location in the assembly to intentionally produce a distortion in a beneficial direction.

They apply the same shrinkage stresses responsible for the original distortion, but in a way that helps overall assembly. Although this is a low-tech technique, it requires a heightened understanding of the forces of distortion and thermal pressure.

Distortion is a part of welding. It cannot be avoided, but by understanding the forces involved, teams can plan to minimize weld distortion. Whether they attack the distortion during the weld or after the weld depends on the situation, but either way, manufacturers must be intimately familiar with the physics and techniques involved in order to mitigate this potentially harmful phenomenon.

Weld Distortion

Predicting Weld Distortion Means Planning For Success

It is critical to work with a manufacturer who can predict and plan for weld distortion. This distortion is a dimensional change in metal caused by stresses that develop during welding. The level of weld distortion can vary depending on how parts are clamped or restrained, how parts are stress relieved, or the choice of welding process.

Unanticipated distortion can slow down schedules, increase costs, or even render a finished product useless. Experienced manufacturers can predict and manage this phenomenon — or at least create a plan to deal with distortion when the welding is finished.

Different Types of Weld Distortion

Weld distortion falls into three main categories:

  • Transverse shrinkage, which occurs across the width of the weld seam,
  • Longitudinal shrinkage, which occurs along the length of the weld seam, and
  • Angular distortion, which results in a bend at a joint.

In most applications, all three occur at the same time in varying degrees. Welding teams must predict which type of distortion is most likely to impact the process, the likely severity of the distortion, the possible ramifications and the best courses of action to mitigate and minimize the effects of distortion.

Predicting Weld Distortion

Manufacturers have two options. They can either take a rudimentary, pen-and-paper approach to predicting weld distortion, or they can invest in sophisticated software to make predictions for them.

Advanced software has drawbacks. First of all, it is expensive, and that cost is inevitably passed on to the customer. Second, predictive software is not as reliable when building one-of-a-kind custom parts that don't have any precedent and may not be made again. Finally, it is easy for builders to develop an over-reliance on software formulas and ignore their hard-earned instincts.

The most reliable method is found in the gray area between the computer and the pen-and-paper. When teams understand the scientific method that software uses to develop simulations, they can integrate those formulas when drawing their own conclusions through the traditional pen-and-paper process.

No matter which methodology is used to predict and mitigate weld distortion, teams must recognize their own limitations or the limitations of their chosen software. If teams treat their conclusion as gospel and design the entire manufacturing process around it, they are setting themselves up for disaster.

Here, experience is irreplaceable and invaluable.

Over time, experienced manufacturers develop gut instincts that tell them to heed or ignore the formula. The formula is a guide, but the best designers will adlib if their instincts tell them to veer off track.

It is important for manufacturers to recognize where reality deviates from the underlying assumptions behind formulas and software. The truly experienced will use the science of welding to find a balance between these rigid formulas and their own intuition.

Addressing Weld Distortion

Once the team predicts the type and severity of weld distortion they are most likely to encounter, it is time to develop a minimization and mitigation strategy.

For some jobs, the solution is stress relief. In other cases, straightening or forming could get the best results. Maybe welders start with a formed shell diameter that is bigger than they need so it can shrink safely into tolerance as it is being welded together. In many applications, it is best to simply start with extra material and then machine it away after welding is complete. In some instances, however, post-weld machining is not an option. Distortion prediction is most critical in these cases.

Any action should be taken with return on investment (ROI) in mind:

  • What is the ROI as it applies to the cost of mitigating weld distortion?
  • What techniques can be reasonably applied to get the most ROI for the amount of work put in?
  • What gives the customer the best value?

In other words, it is not worth adding $1,000 worth of fixturing to avoid spending $500 on post-weld machining.

When metal is welded, it can shrink, bend and distort. This distortion can ruin a project if a plan isn't in place to predict, prevent or mitigate it. The ability to predict and plan for weld distortion is so important that the best manufacturers teach it and conduct periodic training to keep everyone fresh and up to date. In the end, the most powerful tool in the war against weld distortion is experience.