Strengthening and common defects of motorcycle spring steel wire

The factors affecting the quality of the spring are manifold, and the materials and heat treatment have an important influence on their performance and life.

1 The performance of the spring material requires that the yellowing should work in the elastic range, and no permanent plastic deformation is allowed. Therefore, the spring material is required to have a high elastic limit, a yield limit and a tensile strength. Springs generally operate under long-term, alternating load conditions and must have a high fatigue life.

In recent years, the development of buffer coil springs and valve spring materials for front and rear dampers in springs used in motorcycles has been very active.

Since the mass of the spring is inversely proportional to the square of the design stress, the design stress can be increased, and the quality of the coil spring can be reduced by using a small-diameter high-strength steel wire, thereby reducing the cost.

Valve springs with the improvement of motorcycle performance requirements, the material tensile strength ( Methods for improving the strength of the quenched and tempered spring stock are: (1) by adding alloying elements; (2) changing the heat treatment conditions; and (3) strengthening the surface by nitriding.

Methods for improving the fatigue properties of spring materials include: (1) reducing non-metallic inclusions and surface defects; and (2) refining austenite grains.

The methods for improving the elastic damping resistance include: (1) high strength of the spring material; and (2) increasing the tempering resistance of the steel.

The effect of alloying elements on the strength of the tempered material is as shown.

The figure shows the effect of strengthening elements such as C, Si, Cr on tensile strength.

Since the mass fraction of C in the steel is 0.6% or more, the maximum bonfire hardness is basically unchanged, and the C content of the quenched and tempered spring steel is generally 0.5% to 6%, and Si is added for improving the elastic damping resistance and hardenability. Alloy elements such as Mn and Mn, the tensile strength usually used is 1 The influence of the alloying element on the value of the tempering material. The simplest method is to reduce the tempering temperature during heat treatment, but not to reduce it. The fire temperature is used to increase the strength and the chemical composition should be studied. In order to refine the grains, silver is added while increasing the carbon content. Vanadium (V) is dispersed in the carbon (C) and nitrogen (N) compounds to prevent coarse grains of austenite, and the vanadium is returned above 450t. The secondary precipitation hardening at the time of fire improves the tempering resistance, thereby increasing the strength, and the titanium and tantalum are also effective for refining the grains.

The spring material can be integrally reinforced, or it can be surface treated by nitriding to increase the surface hardness and increase the surface compressive stress, thereby improving the fatigue life. It is the effect of alloying elements on the hardness of nitride.

Since the nitriding treatment is a diffusion heat treatment, it is reasonable to increase the curing temperature in order to improve the hardening layer, but as the surface hardness is increased, the mass fraction of the gold added to the motorcycle technology 2002/0115, the alloying element is nitrided. Effect of hardness 2.2 Factors that hinder high strength With the increase of tensile strength, when a certain degree is reached, the tensile strength and the fatigue strength are out of proportion, and the reason is considered to be due to inclusions and surface defects. Refining harmful inclusions is necessary to increase the fatigue strength at high strength, such as reducing the ai23-based oxide. At the same time, strengthening the surface quality inspection and controlling the surface decarburization during the manufacturing process will help to improve the spring quality.

3 Common Defective Springs Affecting the Fatigue Life of Springs Springs are parts that work under varying loads. The form of failure is mainly fatigue fracture. The following are several common spring steel surface defects and microstructure defects that affect the fatigue life of the spring.

3.1 Surface defect folding In the material manufacturing process, if the surface of the blank is not polished, the side skin or the oxidized belt is folded into the wire, and the defect may cause the spring to break during use. After annealing, there are obvious decarburization phenomena on both sides of the fold. The scratches and cracked materials are scratched due to the unsmoothness or insufficient lubrication of the mold holes during drawing. In further drawing, the scratches and cracks extend in the drawing direction and are distributed longitudinally. In severe cases, it will cause fatigue fracture. After annealing, it was found that the side of the crack contained significant shedding.

3.2 Microstructure defects Surface decarburization material processing process after repeated heating and annealing, if the protection is not good, the surface of the material often decarburization, the metallographic structure of the decarburization layer is ferrite, which will reduce the fatigue life of the material.

Free graphite is used for the long-term annealing or repeated annealing of the spring steel with high carbon content and high silicon content to produce graphitization phenomenon. 3 The appearance of free graphite destroys the continuity of the metal matrix and reduces the carbon content in the solid solution. The fatigue life of the spring.

Free ferrite bonfire heating temperature is low, ferrite is not fully dissolved, martensite and ferrite mixture appears after quenching, the presence of free ferrite reduces spring strength and fatigue life. 16 Motorcycle Technology 2002/ 01 The hardness of the core is reduced. To solve this problem, Mo and V are added to improve the temper resistance.

The coarse quenching temperature of the structure is too high to obtain coarse martensite and retained austenite. Large grain size will increase the brittleness of the material, causing the spring to break during use.

The feather-like tissue cools slowly in the quenching cooling medium to form a feathery bainite structure.

The banded tissue raw material has a banded structure, and after quenching and tempering, it still has a band-like trace. Due to the non-uniformity of the structure and composition, stress concentration will be caused in the local weak area, thereby reducing the fatigue life.

4 Spring selection considerations Hardenability To ensure the uniformity of the internal structure of the material, the material should have sufficient permeability. Considering the quality effect of the thermoformed coil spring, Cr is added to improve the hardenability.

Tempering characteristics Adjust the tempering temperature to obtain different hardness, but the change in tempering temperature is not only hardness, but also other properties. For example, when the retained austenite is insufficient in the fire, it still exists at room temperature, and as the tempering temperature increases, the austenite decomposes. When the amount of austenite is large, the endurance is lowered, and the resistance to elasticity is deteriorated.

c Low-temperature annealing cold-formed springs generate residual stress. In order to eliminate residual stress, improve mechanical properties and fatigue strength, low-temperature annealing is required. Low temperature annealing temperature will reduce the tensile strength and fatigue strength of the material, so choose a suitable low temperature annealing. For example, oil quenched and tempered steel wire is annealed at 400 feet.

Electroplating plating can effectively improve corrosion resistance, but hydrogen embrittlement occurs during electroplating, and dehydrogenation treatment (heating and heat preservation of about 200 ft) is required. Hydrogen embrittlement does not easily occur on materials such as string lines.

Materials quenched and tempered by oil quenching and tempering steel wire are prone to hydrogen embrittlement.

The shot peening small metal pellet sprays the surface of the material at a high speed to form a surface strengthening layer of a certain thickness, and a high residual compressive stress is formed in the reinforcing layer, which significantly improves the durability. It has been widely used in valve springs and buffer springs.

The low-temperature soft nitriding surface forms a soft nitriding layer, which significantly improves the durability, and is used for low-temperature soft nitriding of the valve spring to improve the surface strength while ensuring strength.

The effective chemical composition of the elastic resistance weakening property to improve the elastic resistance of the material is Si. The solid solution limit can be improved by the solid solution strengthening of Si: for example, the mass fraction of Si in the Si-Cr oil tempered steel wire is 1.5%, and the buffer spring reaches 2%. In addition, the addition of V refinement grains can also improve the elasticity resistance.

Durability Durability is affected by surface conditions (cracks, decarburization, etc.), decarburization reduces surface hardness, and thus reduces fatigue strength. To prevent decarburization, a controlled atmosphere furnace quenching heating or high frequency heating should be used. (2001-08-29)

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