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Distributor's Link Magazine Summer 2018 / Vol 41 No3


108 THE DISTRIBUTOR’S LINK CARMEN VERTULLO HYDROGEN EMBRITTLEMENT IN FASTENERS CASE STUDIES - PART 3 from page 30 Then they showed me the plastic bag – with our label and the remaining zinc plated spring pins in it. It turned out that we did not have enough plain pins in stock on the previous order and the zinc plated pins were substituted. This was done with the permission of the customer, but no record of the permission was found. The customer had no knowledge of IHE and said if they had known about IHE they would certainly not have used the zinc plated spring pins. The pins were very old stock and there was no traceability for the source of the parts or for the zinc plating process. This fact was not germane to this investigation, but certainly could have been in a more serious fastener failure situation. A very important factor in the investigation was the hardness of the product. Inch spring pins are governed by ASME B18.8.2. This specification calls out the hardness requirement and the requirement for baking of electroplated spring pins. Hardness is HRC 46-53 for high carbon steel spring pins and HRC 43-51 for alloy steel spring pins. Both hardness levels are well into the IHE susceptibility range. ASME B18.8.2 says this regarding plated spring pins: 2.8 Finishes Unless otherwise specified, spring pins shall be furnished with a natural (as processed) finish, unplated or uncoated. Where corrosion preventative treatment is required, steel pins may be cadmium or zinc plated or phosphate coated as agreed upon between the manufacturer and the purchaser. However, where a corrosion preventative finish applied to carbon steel or alloy steel spring pins is such that it might produce hydrogen embrittlement, the pins shall be baked for a suitable time at a temperature that will obviate such embrittlement. After a short conversation we assured the customer we would find out what happened and make it right. In the meanwhile, plain spring pins were provided and the three cut open safes were repaired. Some number of other safes were reworked to replace the zinc plated spring pins with plain spring pins. Fortunately, no safes with the affected spring pins were shipped to any customers. We examined our inventory for other zinc plated spring pins and none were found. I returned to our warehouse with the zinc plated pins and immediately conducted an IHE test on them. There is no standardized test for IHE in commercial spring pins. The easiest way to test them is to simply install them in a properly sized hole that will stress the pin. Wait for a few days. Remove them and check for cracks. Even easier – just clamp the pin lengthwise in bench vise and put enough pressure to slightly close the gap, but not so much as to fully close it. I used the bench vise method. In about an hour I heard the characteristic “tink” sound of an IHE test failure. The pin cracked lengthwise exactly like those installed in the application. As HE failure investigations go this one was quick, straightforward and without serious repercussions. All of the elements of a classic IHE failure were in play here: [1] The material hardness was HRC 46 -53, which is well into the IHE susceptibility range of above HRC 39. In a formal investigation the hardness would have been confirmed by a laboratory test, but in this case it was reasonable to assume the hardness was as specified. [1] There was an obvious source of hydrogen – the zinc electroplating process. [2] There was high load on the steel. Spring pins must flex significantly to be effective. This flexing would impart high stress – not easily measured in a spring pin, but certainly high enough to induce IHE failure in a susceptible product. [3] The failure was delayed in nature –always a prime clue in the IHE investigation. [4] There was no obvious process control to mitigate the IHE risk, such as baking and testing. [5] The fracture surface was brittle in nature. In this case, the fracture surface could appear brittle even if the cause of failure was not IHE – e.g.: excessive hardness. [6] The IHE failure was easily replicated in subsequent testing of samples from the same lot of product. [7] No understanding or awareness for the risk associated with electroplating high hardness fasteners was evident on the part of the end user or executed upon on the part of the supplier. Fortunately, the repercussions of this IHE failure were not significant. The customer asked for a few thousand dollars for their repair and rework costs, which I was readily willing to pay. Our company owner, a more savvy business person than I, offered half of what they asked for as a credit against future business. The customer accepted that offer. CONTINUED ON PAGE 168



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