Hydrogen Embrittlement: How Small Details Can Have Large Effects
Knowing the facts about hydrogen embrittlement from the beginning will save both time and money down the road.
Hydrogen embrittlement - an ugly plague that cast its shadow on the steel industry and was identified almost 150 years ago - is the same culprit that has claimed responsibility for the majority of nautical and aerospace/aircraft disasters in the last 40 years. It causes what is known as catastrophic fracture failure. This refers to an instance in which a steel structure has fractured due to a loss in strength and ductility - or the ability to bend. The reason for this is quite logical. The steel fractures as a result of an overabundance of cracks in the structure. Every structure is going to have some cracks somewhere - this is inevitable. However, because of hydrogen embrittlement, primarily high-strength steel structures such as fasteners can fail prematurely and abruptly, causing many complications and, in some cases, disaster.
But What Is It?
Most people in the industry have heard the term hydrogen embrittlement, but not everybody may be aware of its seriousness. Hydrogen embrittlement is a brittleness of metal and chrome layer structures, resulting from the occlusion of hydrogen or condition of low ductility, which itself results from hydrogen absorption and the internal pressure developed subsequently. Basically, the hydrogen is interfering with the metal's ductility, which in turn makes the chrome build-up weaker and greatly increases the chances of breaks or cracks in the chromed area.
But what do damaged airplanes and ships have to do with the moldmaking industry? Just as a disaster could occur to an airplane, similar disasters also could occur in molds. When dealing with a part that will endure at least 140 psi, one should consider the risks of not acknowledging hydrogen embrittlement and the detrimental effects it could have on the molding process. While it could affect molds and mold inserts enduring less than 140 psi, hydrogen embrittlement is more likely to occur with a structure that is bearing a heavier load.
It also is important to keep in mind that the hydrogen embrittlement becomes more of a threat as the amount of chrome applied increases. In a build-up - one in which you are applying layers of chrome - hydrogen becomes trapped between every layer of chrome applied, thus increasing the chances of hydrogen embrittlement. In a flash chrome situation - one in which a minimal amount of chrome is applied - the probability of this problem occurring becomes less likely because the hydrogen doesn't have as many chances to embed itself within the structure.
Electroplating is a cost-effective way to protect your molds from corrosion while increasing releaseability. It is a widely used process. When someone pays money to have a mold plated, it is an investment for the long run. It is an investment in which they trust the application and service of the chrome platers to prepare their mold to be able to endure many applications of plastic.
The mold and die industry is one founded on teamwork. Parts of the team include mold builders, heat treaters, polishers, molders and even the store at which the parts are sold. All of these people rely on one thing - the quality and efficiency of the parts. All it takes is for one mold to crack, or fracture, and all of these people are delayed. In an industry that is based on efficiency, time is money. Understanding hydrogen embrittlement and its effects on the mold and die industry is important when looking at money flow and worker efficiency. The more you can get done right the first time saves you money in the long run by avoiding that rework order. This is why people chrome plate parts in the first place - to avoid the maintenance of an unprotected one.
The real problem is that hydrogen is everywhere, being a simple atomic element. It readily bonds with the base metal in the structure and embeds itself in the actual makeup of the part. But when does this "infection" occur?
When preparing a part for electroplating, the steel needs to be ground and cleaned - usually with an acid. The mineral acid becomes absorbed into the steel, which is then put into the tank to be chromed. In electroplating, only about 20 percent of the electricity used in the process is utilized to apply the chrome. The other 80 percent of the electricity helps to create a side reaction called hydrolysis, which is a chemical process of decomposition involving the splitting of a bond and the addition of the hydrogen cation (a positively charged ion) and the hydroxide anion (a negatively charged ion) of water.
Unfortunately, this hydrolysis is unavoidable during the electroplating process. When chrome plating, the chrome is applied in layers to assure the proper buildup of chrome throughout the piece. However, this means layers of trapped hydrogen, which at the molecular level is creating hydrogen blisters. When the piece is removed, that hydrogen wants to escape, and does so by creating micro-cracks in the surface - greatly impairing the strength of the piece.
It is important to understand that it is not because of the plating process that these parts are being affected by the hydrogen embrittlement, but it is the way in which the pieces are handled before and after the plating process.
Hydrogen embrittlement is putting an overabundance of stress on the steel, so it makes sense to say that a "stress relief process" to alleviate this unwanted tension is very necessary. Timing is crucial at this point. It is imperative that within one hour of a piece's withdrawal from electroplating that it be put through the relief process. One hour is the maximum permitted wait time - but in dealing with this realistically, sooner is better. What the stress relief process does is basically temper - heat or bake - the piece to allow the hydrogen that is trapped within the chrome to escape with virtually no stress, cracking or weakening, ensuring a strong, efficient part.
The time a piece remains in the oven varies. Usually a piece will be in no less than three hours. However, depending on the actual make-up of the piece, the size and the amount of chrome applied, it could be in there for up to 24 hours or longer. The harder the steel, the longer it should be in the oven. Steel with tensile strength of 1,000 would be treated for about eight hours. For every 2,000 MPa increase, one should increase the oven time two hours. The heat at which the structure is baked at should be no less than 375'F and closer to 400'F.
This process is extremely necessary to alleviate the potential problems that could arise due to hydrogen embrittlement. You cannot avoid it - although many people do try to ignore the reality of hydrogen embrittlement and do not put the pieces through the stress relief process. This only leads to frustration, misunderstandings and pointed fingers and reflects poorly on chrome plating as a process. For example, when trying to build up the edge of a piece with chrome, every time it was taken out it would crack, thus causing more time to be spent on it than was necessary. With all of the layers of chrome that were being applied, the accumulation of excess hydrogen was obvious. This accumulation was what was causing the chrome to crack. The hydrogen was trapped within the layers of chrome and needed to escape. Once this was realized, the part was put through the stress relief process immediately following the plating. The cracking stopped, and the problem was solved.
Knowledge Is Power
It is important to realize the harmful effects of hydrogen embrittlement, and to know how to prevent them from affecting the customer's productivity in the long run. Whether you are flying an airplane, molding plastic parts or dealing with high-strength industrial steel, you should understand that hydrogen embrittlement plays a part in all of these. The only way to relieve the stress caused by this accumulation of hydrogen is through a heated process specialized for this one problem. When looking for a supplier, make sure that they not only know the facts about hydrogen embrittlement, but also how to keep it from emptying your pockets. Through reworking and trial and error, the service of chrome plating can get expensive. Find a place that will do it right the first time. Save yourself the time and sanity. You and your supplier should both have the same number-one priority - long-run efficiency and customer satisfaction.
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