Almost all welders are familiar with the concept of electroslag welding, and with good reason, but we will discuss that in a bit. First of all, let's talk about the idea behind electroslag welding:
The Concept
For thick materials, electroslag welding is perfect. This process is considered to be highly productive, especially for a single pass technique. During the process the electric arc is struck by the wire which is fed into the proper weld location, and then flux will be added. As flux is added, the molten slag will reach the tip of the electrode and extinguish the arc. The wire will be continuously fed through, and both the wire and tube will move along the workpiece. A copper shoe will be utilized during the process to keep the weld between the plates while the welding takes place. It is well known that this form of welding requires quite a bit of heat, and in some cases the copper shoe will actually be water cooled. Depending on the thickness of the material being welded, a higher voltage may be required. Because the arc is extinguished before the process begins, this cannot be considered a form of ARC welding.
The Origin
The concept and process of electroslag welding was patented by Robert K Hopkins, an American, in February of 1940. For those interested, the patent number is 2191481, the actual process was refined at the Paton Institute at Keive in the USSR throughout the 1940's, just before America became involved in what would become the Second World War. As with many technological developments of the time, electroslag welding was ultimately shown off at the world's fair where it was picked up by several automobile manufacturers. For instance, General Motors would use it to weld engine blocks, and it would even be used in the construction of certain buildings. One shining example would be the Bank of America building in San Francisco, along with the Security Pacific building in Los Angelas. A number of different buildings were used, and after the Northridge earthquake, a number of different buildings needed repair; the electroslage welding process was used to take care of these issues. It should be noted that there was not a single weld failure on any of the repair jobs, which is actually quite impressive in a new process.
The Banning of Electroslag Welding
In 1977 the Federal Highway Administration decided that it would take an active role in the use of electroslag welding on primary structural tension members. That is to say it was made illegal for use on bridges, and in many cases it's use on building supports was ceased as well. The ban stayed in place for nearly twenty years, and many were interested in the reasons behind this action.
The problem that the Federal Highway Administration noted was the fact that the amount of heat required to create a good weld would also produce course weld metal. The theory was that this would lead to serious fractures, and would at some point cause a structural collapse. The biggest problem however is a severe lack of understanding regarding the process which many believe was the reason for the banning in the first place.
A New Hope for Electroslag Welding
As of late, new methods of electroslag welding have been tried, and Narrow Gap Improved Electroslag Welding (NGI-ESW) was created with the hopes of meeting the toughness requirements of Zone 2 bridges. It should be noted that Zone 2 bridges are defined as those that open under friction, which generally means modern day drawbridges that are found on rivers with high traffic. It is understandable that these will require a tougher weld, and the NGI-ESW process has produced favorable results. One of the greatest advantages to NGI-ESW is the fact that there is no pre-heating required. The major disadvantage is that the process needs to be performed in the vertical-up position. That being the case, the maximum length of the weld is approximately four meters.
Whether or not there was ever truly a problem with electro-slag welding in the first place is anyone's guess. There are few, if any failures related to the process, though there are always those accidents that go unreported. Some failures may be attributed to welder incompetence, but in the end, only the law of physics can really make the determination.