WINTER 2016

26 THE DISTRIBUTOR’S

26 THE DISTRIBUTOR’S LINK Carmen Vertullo Lead Trainer, Fastener Training Institute ® FASTENER TRAINING INSTITUTE ® 5318 East 2nd Street #325, Long Beach, CA 90803 TEL 562-473-5373 FAX 661-449-3232 EMAIL info@fastenertraining.org WEB www.fastenertraining.org TO BAKE OR NOT TO BAKE? Sooner or later most suppliers of electroplated fasteners get around to asking and answering this question. Unfortunately for many the answer comes too late because the question comes too late. They have already experienced a hydrogen embrittlement failure event as the result of a half-baked baking strategy, or they have wasted untold thousands of dollars baking plated fasteners that did not need to be baked. This article is not about hydrogen embrittlement, so when you finish reading it you will not know all about hydrogen embrittlement. This article is about baking, so what you will know is how to make the best decision about embrittlement relief baking from a risk management perspective - and if you do want to know all about hydrogen embrittlement in fasteners you will have access to the best and most current technical resources available to the fastener supplier, manufacturer, or user. The most current research and experience is not reflective of many older specifications which most platers, users and suppliers are following. However, some standards are up to date and there are some new resources that we all need to be aware of. Of course there is a little more to the question than just “to bake or not to bake”. Some of the more obvious rejoinders are: Why do we bake? At what temperature? What fasteners require baking? What fasteners do not require baking? Which ones, if any, are borderline? What processes require baking? What processes do not require baking? For how long do we need to bake? • When in the process sequence should the baking be done? TECHNICAL ARTICLE Who decides? • How much does it cost? What specifications address baking? How do I know if the baking was effective or not? Before we attack these questions, just to get us all on the same page let’s get a basic definition of Hydrogen Embrittlement (HE) as it relates to fasteners: Hydrogen Embrittlement: Certain metals, such as carbon steel and alloy steel may absorb atomic hydrogen which is generated in processes such as acid cleaning, pickling, and electroplating. When higher-strength fasteners are placed under a load, (tightened above a certain percentage of their yield strength), the hydrogen migrates in the metal to the areas of high stress, usually at the head-body junction or the first loaded thread. The hydrogen concentration at these high stress locations causes micro-cracks to form. The micro-cracks become larger cracks which eventually (usually in a few hours to a few days’ time) leads to catastrophic failure of the fastener. This defines a type of HE that is process induced and it is called Internal Hydrogen Embrittlement (IHE) because the source of hydrogen is internal at the time the fastener is put under a load. Another type of HE is caused when the source of hydrogen is external to the fastener under load, usually caused by corrosion of the fastener, of its coating or of the assembly in the environment. This type of HE is called Environmental Hydrogen Embrittlement (EHE). The mechanisms involved in these two types of HE is the same, only the source of the hydrogen and the timing is different. Because this article focuses on the HE relief baking process, only IHE is of concern here and we will refer to it as simply HE. CONTINUED ON PAGE 114