Development of steel wire for valve springs: For the manufacture of ultra-high stress valve springs with stress levels ≥1000MPa or more, the basic requirements are as follows: 1) High purity: In addition to P and S, the gas elements present in steel, such as O and H, are also strictly limited. 2) Adding a beneficial element that promotes nitriding: In order to allow the material to stay at a relatively high temperature (460 ° C ~ 480 ° C) for a long time, the hardness of the substrate can be maintained above HV 580, Si is an indispensable element. Cr is a commonly used additive element in spring steel and also promotes one of nitriding elements. Mo is both a beneficial element in spring steel and an element that promotes nitride formation. V can effectively refine grains and promote nitrogen. Therefore, Cr, Mo, and V can all be included in the list of added elements. Although Al is a strong nitride element, it is a major factor in the formation of B-type (alumina) inclusions and cannot be used as an additive element. 3) C and Ni content: C is a key element to ensure the strength (hardness) of the valve spring material. After the carbon content in the steel reaches 0.6%, the effect of increasing the carbon content on the strength (hardness) is not obvious, so the average carbon content is controlled at 0.6%. The addition of Ni is beneficial to improve the toughness of the material. (2) Middle and high end valve spring wire The high-end valve spring wire is suitable for the manufacture of valve springs with a stress below 1000 MPa, of which Si-Cr is still the main additive element. Considering the cost factor, two kinds of steel grades are designed for the manufacture of valve springs with stress τmax≥900MPa~1000MPa and τmax≤900MPa. The amount of material composition and alloying elements added is as follows: C: The high value of carbon content of valve spring steel wire with stress τmax≥900 MPa~1000MPa is 0.56%~0.64%; the carbon content of valve spring steel wire with stress τmax≤900MPa is medium level, 0.51%~0.59%. Si: An element that can effectively increase the yield ratio of the material, and weighs the favorable (enhanced matrix) and harmful (graphitization) factors of Si, and the content is 1.20% to 1.60%. Mn: weak carbide element, the grain refining effect is not great, the main role of the addition is to reduce the hot brittleness of sulfur, and it can be added to 0.50%~0.80%. Cr: a medium carbide element, which forms a carbide to increase the strength of the matrix and suppress the tendency of Si to graphitize. From the cost considerations, Cr is 0.9%~1.2% for V2 and 0.60%~0.90% for V3. V: The effect on the grain refinement is very obvious, and the valve spring wire must be added. The amount of V2 added is 0.10%~0.20%. Considering the cost factor, V3 steel will reduce V to 0.05%~0.15%. B: Boron has the effect of purifying grain boundaries, and the addition of a trace amount of boron serves to suppress the harmful effects of impurity elements in the segregation grain boundaries. 4 High-stress valve spring steel wire development related issues (1) Billet smelting Valve spring fatigue cracks often originate from the surface of materials or near-surface inclusions, so there is a high requirement for the purity of valve spring billets, and must be ultra-pure. In addition to strict restrictions on the impurity elements S and P, the harmful gas elements O and H should be strictly limited, and Al and Ti form non-deformable inclusions, which are also limited. Restriction requirements for harmful elements: S ≤ 0.020%; P ≤ 0.020%, strive to control below 0.010%; O content control below 30ppm; H is not allowed to appear; Al content is below 0.05%; Ti content is below 0.03%; Up to 0.06% or less; other harmful impurity elements such as As, Sb, Sn, etc. are controlled accordingly. For refining outside the furnace, V1 steel considers electroslag remelting; V2 and V3 steel wire adopt vacuum degassing process, and the feasibility of the two process schemes still needs to be demonstrated. (2) Hot rolled wire rod The ingot is used in the ingot, and the steel billet is subjected to peeling and flaw detection before rolling; the quality of the wire rod is not folded, the surface decarburization is ≤0.07; the surface defect is ≤0.10. (3) Steel wire processing 1) Surface peeling: the stripping amount of the strip strip is not less than 0.15mm on one side; 2) The surface of the steel wire has no full decarburization, and the partial decarburization depth is not more than 0.5% d. The steel wire passes through two (through-and-rotary) eddy current testing, and the surface defect depth of the material does not exceed 40 μm; 3) Mechanical properties of steel wire: It is related to the thickness of steel wire. The finer the steel wire, the higher the strength of the material. (4) Spring nitridation process 1) Development of spring nitriding equipment; 2) Nitriding process: low temperature (440 ° C ~ 480 ° C) nitriding atom diffusion rate, heating mode (segment or segmentation), furnace temperature difference, nitriding time, atmosphere composition and active nitrogen atom incidence (such as NH3 decomposition Rate) the method of furnace loading, the uniformity of the furnace atmosphere, and the influence of the thickness of the layer; 3) The hardness of the nitride layer is not less than HV800, the thickness is not less than 1μm; the hardness of the core is not lower than HV580; 4) Shot peening process, number of times, pellet size and hardness before and after nitriding. Pendulum Impact Test Machine,Pendulum Impact Tester,Izod Impact Testing Machine,Display Charpy Impact Testing Machine Dongguan Best Instrument Technology Co., Ltd , https://www.dgbestinstrument.com
(1) High-end valve spring wire