Defects in Hot Forgings

Dec. 14, 2005
Defects can be defined as imperfections that exceed certain limits. Here are some facts about common and not so common forging defects that come from actual forging operations or from post forging operations typical of many forge plants.

When we think of all the ways of making a part from metal, forging stands out as the best way to achieve top quality and performance of vital parts. Forgings sometimes cost more than parts produced by other means, e.g. casting, powder metal, weldments, etc. But forging quality is worth it to the designer if he continues to experience high reliability from forged products. As a commercial says, he “expects more and he gets more.”

Or does he? Sometimes forgings are made improperly and fail in service. The customer becomes unhappy with not only the forging supplier, but also with his choice of forging as the production process. When a product fails, there not only is the loss of customer confidence in selecting forging for the next critical part, but there is also the specter of product liability.

The forging company’s quality system may have failed or someone may have failed to practice good workmanship. It has been proven time and again that systems do fail no matter how elaborate the inspection procedures.

To make matters worse, if one forge shop fails to consistently meet good quality standards, it can blemish the reputation that all forgings have for reliability.

A good quality program begins with an attitude — an attitude of making it right the first time, of asking “what can I do to help?” This is pride of workmanship.

Most employees sincerely want to display workmanship and they do a better job if they know and understand why they are doing what they are doing. Quality conscious operators stop and ask when they see something unusual happening during forging, especially when then feel encouraged to ask an “informed question.” Untrained employees may not show the desired interest.

This discussion deals with the kinds of defects that form during the forging process. They sometimes are traceable to the starting material but more often to the forging process itself. The discussion that accompanies each description of a defect provides possible cures for that type of defect. In some cases, there is more than one cause. Options are discussed for each type of material or forging process.

Forging defects: a review

Forging defects are not widely discussed in the literature because of a natural reluctance by forgers to draw attention to them. There are many imperfections that can be considered as being defects, ranging from those traceable to the starting materials to those caused by one of the forging processes or by post forging operations.

Defects can be defined as imperfections that exceed certain limits. In other words, there may be imperfections that are not classified as true “defects” because they are smaller than allowances in the applicable specifications.

A good case in point is the comparisons between air-melted steels and vacuum arc re-melted steels. A magnetic particle indication in air-melted steel may be as long as 1/4-in. and still be acceptable under certain conditions. Yet, such an indication size in vacuum arc re-melted steel would be considered a defect. As a matter of fact, air-melted steels have become cleaner and cleaner over the years and most heats produced now are able to meet the older aircraft magnetic particle standards of the 1960s and 70s. This is an example of how continuous improvement efforts at the steel mills are paying off.

Similarly, there are differences in allowable imperfections on the surfaces of forgings and these vary depending on the material being forged. This is one area that requires attention especially as a focus on continuous improvement.

The point being made here is that it is important to move any process in the direction of eliminating all imperfections as part of an effective continuous improvement program. Forging processes are no exception to this.

Furthermore, forgings represent what should be the ultimate product in terms of performance compared to other parts-making methods.

Some forgers can unknowingly or unintentionally ship defective parts. This would be very unusual for companies having well-trained personnel who have superb workmanship attitudes.

As noted, when this does happen, the customer may be unhappy not only with the source of the forgings, but he also may become unhappy with the choice of forging as a process that is not living up to his expectations. Thus, all forging companies have a stake in the situation when and if bad parts are shipped by any one forge plant. Industry-wide attention and commitment are needed here.

Commitment from all employees is necessary to accomplish this goal. Management cannot direct or mandate superb forging quality. Managers must lead people to produce superb quality through continual education and training of employees at all levels.

The following material is aimed at providing information about common and not so common forging defects that come from the actual forging operations or from post forging operations typical of many forge plants. The goal here is to acquaint the reader with these various defects, with how they can affect forging performance, and how to eliminate them with future forging production.

Starting material defects

Many forging defects originate from the starting material, including surface seams, out-of-tolerance cross-sections, and center conditions (carbon segregation, pipe, etc.). These defects are not discussed in detail here because they do not originate in the forge shop. On the other hand, poor center quality can lead to ruptures during forging, and seams can lead to peripheral cracks on parts during upsetting. Thus, an awareness of such material defects should be taught in any on-going in-plant training efforts.

Seams—These trace back to the billet or bar mills where the final product comes from rolling mills that do not have clearly defined ways of conditioning the material in process. Very long continuous seams usually come from the multiple passes in the billet or bar mills and where the product of one pass can actually create folds on the product that eventually are rolled in to form seams or long defects on the bar/billet surfaces. During upset forging, seams can lead to peripheral splits on the resulting forging.

Internal Cracks —These often trace back to the ingot practice at the steel mill and whether there is enough hot top removed during conditioning. This can also be traced to continuous cast billets that are solidified too quickly, or when ceramic materials from the tundish or nozzles break off and drop into the mold area and become trapped in the continuously solidifying liquid metal, then become part of the center segregation. Steel mills having the larger bloom-sized casters are able to achieve center quality levels equal to or superior to ingot cast billet.

There are other internal defects that can come from the forging operations including internal cracking or from the heating operations including overheating and burning.

A variety of defects that come from forging operations, as well as from starting materials, and their causes, cures, and other actions are discussed in the tables “Surface Defects: Causes and Cures” and “Internal Defects: Causes and Cures.” The emphasis is on impression-die forged parts.

In the final analysis, these listed conditions represent the kinds of defects that should never get out of the forge shop.

If these types of defects do get out, the customer might turn to other processes, meaning lost business and lost jobs for the entire forging industry, and not just your company.