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Analysis of the cause of torsion spring rupture
Sep 07, 2018

Fracture Reason: In the early stage of electroplating zinc, the torsion spring can produce abnormal martensite, because of the microstructure stress of martensite, and the interaction of the internal stress caused by hydrogen in the spring substrate during pickling and zinc electroplating, the torsion spring cracks and lag fracture.

Spring steel wire production of torsion springs before the customer assembly found a small number of spring break, as shown in Figure 1, the fracture position as indicated by the arrow.

Torsion spring production process: spring steel wire → winding spring → low temperature stress annealing → high temperature oil removal → washing → dilute hydrochloric acid washing → washing → zinc plating (min) → washing → blanking → hydrogen treatment (200℃,4 h) → feeding → washing → color passivation → washing → drying → blanking → inspection. Through the analysis of microstructure and microhardness, it is found that the metallographic microstructure of the spring fracture and the nearby crack is martensite. Due to the large stress of martensite, it is easy to form stress concentration area, and the hydrogen embrittlement sensitivity of martensite is larger than Bainite and pearlite, and hydrogen-induced along crystal fracture is prone to occur [4-5]. The formation of martensite should be in the early electroplating of zinc due to the spring and the electrode between the arc, causing the spring to produce electrical burns, the instantaneous high temperature in the electrical burns over the austenite temperature, and then in the electroplating solution of the cold, so that the torsion spring produced abnormal martensite tissue. In addition, the torsion spring in pickling and electroplating zinc process, inevitably there is hydrogen permeation process [6], a part of the precipitated hydrogen in the form of hydrogen molecules escape the surface, the other part adsorbed on the spring surface, and the spring substrate diffusion. The hydrogen atoms entering the matrix gradually accumulate in the place of dislocation, grain boundary, inclusions and so on, which are combined to produce the hydrogen molecule, with the increasing of the concentration of hydrogen molecule, resulting in lattice distortion, resulting in great internal stress [7]. Due to the presence of a high concentration of hydrogen in the spring matrix and the martensite interaction in the process of zinc electroplating, the torsion spring cracks and causes the lag fracture.

Cracks and fractures result in a zinc-plated layer falling off between the coating and the substrate.

Recommendations for production Process improvement:

(1) Torsion spring pickling, to prevent over-corrosion, the corrosion inhibitor added in the pickling solution must have a strong corrosion inhibition, but also strong resistance to hydrogen permeability. (2) electroplating zinc process, strict operating procedures to prevent the emergence of martensite;

In the premise of guaranteeing the quality of the coating, the time of zinc electroplating should be shortened.

(3) After electroplating zinc, as far as possible to shorten the time interval after plating and de-hydrogen treatment, the use of effective hydrogen removal process. (4) Improve the electrode protection measures to avoid the generation of arcing.