First of all, we must know that the heat treatment of automotive hardware fasteners is performed at high temperatures. In order to reduce the formation of surface oxidation during heat treatment, a protective atmosphere is often added to the heating. If the protective atmosphere contains oxygen compounds (such as methanol cracking gas, Rx gas, etc.), the bolt may absorb hydrogen during the heat treatment and increase the risk of hydrogen embrittlement of the bolt.
Research shows that the actual use of threaded hardware fasteners in the natural environment of hydrogen embrittlement fracture is quenched and tempered martensitic steel, occurs in yield strength> 620Mpa, hardness> 31HRC high-strength material. The higher the tensile strength, the more sensitive to hydrogen embrittlement, the easier it is for the material to absorb hydrogen, and it is not easy to drive hydrogen.
In order to prevent hydrogen embrittlement of 1000-1300Mpa high-strength hardware fasteners, the hydrogen flooding process must be standardized after plating. Hydrogen flooding utilizes the reversibility of hydrogen in metals to hydrogen-treat hydrogen-sensitive materials. During hydrogen flooding, the electroplating bolts are heated to a certain temperature for a period of time to allow the hydrogen in the material to accumulate to form hydrogen molecules and escape. Hydrogen flooding is not one of the important causes of hydrogen embrittlement of bolts. There are two reasons for the incomplete hydrogen flooding. One is that the hydrogen is not driven in time after plating, and the other is that the hydrogen flooding time is too short.
It is generally believed that hydrogen embrittlement fracture occurs when the strength reaches 1050 MPa or more. GB/T 5267.1 “Fastener Electroplating Layer” stipulates that when the core part or hardness is more than 320HV, hydrogen embrittlement should be tested through experiments and hydrogen flooding is required. That is to say, plating bolts with a performance rating above 10.9 should use low hydrogen technology and drive hydrogen. According to recent foreign data, the intensity threshold of hydrogen brittle fracture in automotive hardware fasteners may drop to 1000 MPa.
The influence of metallographic structure on the hydrogen sensitivity of carbon steel and alloy steel is obvious. Tempered martensite, upper bainite (crude), lower bainite (fine), sorbite, pearlite, and austenite reduce the sensitivity to hydrogen in turn. Tempered martensite has the strongest sensitivity to hydrogen embrittlement. Therefore, automotive hardware fasteners can adjust the heat treatment system to reduce the formation of tempered martensite during heat treatment. For example, the use of isothermal quenching will increase the threshold for the occurrence of hydrogen embrittlement fracture by about 100 Mpa. This is because lower bainite microstructures produced by austempering are less sensitive to hydrogen embrittlement than tempered martensite.
Manager: Allen Liang
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