Electroplating is a conventional process for spiral springs coating. One of the major problems in this process is considered to be the undesirable reaction leading to hydrogen embrittlement. Taking into account the application of springs in dynamic conditions, any ductile reduction may cause sudden and quick fracture. Unfortunately, hydrogen embrittlement in steel springs will not come along with any special signs. In addition, evaluation of hydrogen penetration and its consequent embrittlement requires very complex laboratory works, and is time consuming with little repetition due to its being affected by surface conditions as well as steel springs variations.
In the present scheme, based upon a simple tension test, it would be possible to evaluate hydrogen embrittlement and standardize its allowable range as per the tolerable bending angle of steel spring. For this purpose, a U-shaped steel part has to be manufactured and electroplated. Thereafter, the aforesaid U-shaped part will be put under tension up to the fracture point by means of a simple table tensioning device and the axial length alteration percentage of the sample spring will be measured precisely. Multiplying the opening angel of U-shaped sample before fracture by the percentage of length reduction along the sample’s axis, the resultant value will be the sensitivity index to hydrogen embrittlement. In this design, the base for evaluation of hydrogen embrittlement severity is the deformation measurement of steel spring after electroplating. In other words, those samples with fractures of more than the allowable range are proved to have excess hydrogen embrittlement severity.
Hydrogen embrittlement is one of the hydrogen attacks occurring at low temperatures. This phenomenon occurs when atomic hydrogen is compiled inside steel through cathodic absorption. Following this, steel will become brittle and breaking and its softness and toughness will be reduced. Hydrogen embrittlement occurs along with the downfall of steel tension strength . It is proved that more strength in metals and alloys under hydrogen penetration will lead to quicker cracking and fracture as the result of hydrogen embrittlement . Hence, in some of the scientific reports, it is seriously forbidden to place high-strength steels under hydrogen-contained environments such as electroplating bath . Being placed under simultaneous effects of hydrogen-containing and mechanical loads environments, steel products will be susceptible to the hydrogen embrittlement sudden fractures and their cracking sensitivity will increase. This embrittlement is caused due to the capability of hydrogen molecules to be converted into hydrogen atoms near the metal surface, which will come along with hydrogen atoms penetration to the alloy. Hydrogen embrittlement has several mechanisms, all of which are related to the hydrogen interplays in metal structure [3–5]. One of the most important of these mechanisms is tension resulting from hydride formation [4–5], discontinuity model , and embrittlement due to the local plastic deformation .
Lots of laboratorial and theoretical efforts have been made to evaluate and measure hydrogen embrittlement values. In all of these, the base for study and measure hydrogen embrittlement was to evaluate the amount of changes in metal mechanical properties as the result of hydrogen embrittlement. In other words, as hydrogen embrittlement has no effects on the metal appearance, the only way to study, examine, and evaluate would be the assessment of alteration amounts created in the metal properties due to hydrogen embrittlement [8–11]. However, it is necessary to indicate that lots of metallic specifications as like microstructure, defects density, accurate chemical composition, hydrogen concentration in the metal, etc may affect the level of hydrogen embrittlement, while these specifications are not taken into consideration in mechanical tests [12–14]. More clearly, the aforesaid specifications are not examined in a mechanical test such as tension. Therefore, outspread results may be observed in mechanical tests regarding hydrogen effects on hydrogen embrittlement severity, and quantitative analysis of the relevant results may indicate so many differences. This is mostly due to the hydrogen embrittlement as a phenomenon in which atomic scale does not happen, while most of the laboratorial works are available in macro scales based upon hydrogen embrittlement mechanical evaluation [15, 16].
Steel springs are extremely affected by hydrogen penetration and become susceptible to the hydrogen embrittlement phenomenon in electroplating process. In the present article, a scheme has been proposed practically using a simple tension test based upon ductility of a U-shaped sample in order to evaluate the severity effects of hydrogen embrittlement on the mechanical properties downfall, especially the final tension strength. This method is very simple with low costs, applicable to all the spring materials particularly steel springs.
Materials and research method
For a better and more accurate explanation of this phenomenon regarding the evaluation of hydrogen embrittlement value based on the introduction of embrittlement index in tension test, it is necessary to consider one of the most specifically usable steel spring materials and perform the required tests on that. For this purpose, carbon steel spring with standard code DIN 1.1230 (of Oil Temper spring type) was applied in this study. Relevant tension strength is 1670 MPa and the chemical composition is provided in table 1.
A few samples were manufactured with specific dimensions and geometry out of this carbon steel spring, as per figure 1. The aim of this selection was to do the consequent tests more feasible.
Now it is possible to perform electroplating on these samples to a desired quantity under actual plating conditions along with the main springs of the same type that are going to be coated. Plating was done simultaneously on the test samples and main springs. Electroplating element was Zinc (Zn), during which a coating with 10 µ thickness was made on the samples. Chemical composition of electroplating liquid as well as the bath conditions is indicated in table 2.
Calculation of the sensitivity index to hydrogen embrittlement
In this test, five samples with geometrical conditions as set out in figure 1 along with the main springs were electroplated with Zinc. Then, the samples were put under tension test by means of a simple tensioning device that is mostly used to determine spring force in different lengths. Figure 2 displays a schematic of force application to the U-shaped sample in tension test. The above-mentioned tensioning device with the capacity of 500 kgf is displayed in figure 3. Test procedure is as below: The U-shaped sample was fastened between two holders and firmly under tension. As the result of this force, the mouth of the sample started opening and the test was continued through increasing the tension force. This had to be continued until the complete fracture of samples. As the perpendicular distance from fracture point to the elongation of actual applied force for various samples is different at the fracture time taking into account their deformation value, this distance for each sample was measured via placement of fractured parts beside each other (fig. 4). Afterwards, according to the initial distance from fracture point to the force application elongation displayed in figure 1, it will be possible to calculate the reduction percentage of this length as the result of tension test until fracture moment of the sample.
On the other hand, the degree of the opening angle before fracture is easily measurable through placement of fractured parts beside each other. Multiplying this degree by elongation reduction percentage of the aforesaid perpendicular distance, sensitivity index to hydrogen embrittlement will be reached. The smaller index shows less deformation in tension test and more hydrogen embrittlement severity.
According to the above, table 3 displays the results of these measurements for the calculation of sensitivity index to hydrogen embrittlement of the samples. It is necessary to indicate that five samples are selected in this study to increase the accuracy and finally an index has to be reported as the average. As it is evident in table 3, average amount of sensitivity index to hydrogen embrittlement is 0.2178 for this instance. The smaller sensitivity index shows more hydrogen embrittlement with less deformation tolerance. This is consistent with applied forces until the samples fracture. In other words, more brittle samples with smaller indexes have also been fractured under smaller forces. This is quite logical due to the tension strength downfall in steel spring as the result of hydrogen embrittlement.
- As the hydrogen embrittlement mostly affects the final tension strength of steel and reduces the steel ductility in a tangible way, we have multiplied angle alteration of U-shaped sample until fracture by the reduction percentage of perpendicular distance of breaking point to the force application elongation in order to calculate sensitivity index to hydrogen embrittlement. The higher index value means the smaller hydrogen embrittlement severity.
- It is possible to apply this method to examine the influences of different operational factors such as temperature or time of dehydrogenation through tempering on the hydrogen embrittlement severity. In other words, the embrittlement index can be defined for various temperature and time conditions.
- To apply this phenomenon in a practical way, a specific spring material has to be electroplated under optimal conditions and U-shaped samples must be plated under the same circumstances. Thereafter, sensitivity index to hydrogen embrittlement has to be calculated through the aforesaid procedure. This is the index gained under optimal conditions. At the time of electroplating a series of these springs, we will be able to electroplate a U-shaped sample along with the main springs and determine the sensitivity index and finally compare the same with the electroplated sample under optimal conditions. The result will be the evaluation of hydrogen embrittlement severity in a comparative way.
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