Elsevier

CIRP Annals

Volume 69, Issue 1, 2020, Pages 281-284
CIRP Annals

A testing method of cold forging performance of steel wires

https://doi.org/10.1016/j.cirp.2020.04.041Get rights and content

Abstract

The flow stress of wire material, the friction coefficient and the sticking capability of the lubrication coating on the wire surface are evaluated with a single test. In this test, the specimen cut off from the end of a wire coil is set on a V-groove die and then is compressed into the V-groove by a punch. Friction coefficient is determined by the ratio of the punch load and the die load. The flow stress curve is calculated using the punch load-punch stroke curve. The proposed method is suitable for the use in the wire manufacturing shops for cold forging.

Introduction

In cold forging industries, steel wires account for a large percentage of starting material and are formed to bolts, nuts, shaft parts, etc., by headers and formers. Steel wires for cold forging are finished by wire drawing after lubrication coating. Thus, the forging performance of steel wires is influenced by the flow stress of the wire material, the friction coefficient and the sticking capability of the lubrication coating on the wire surface.

Some testing methods to measure the forging ability of billets have been developed. The upsettability test [1] was designed to measure the ductility of a billet material. The uniform compression test [2] and the constraint compression test [3] were proposed to obtain the stress-strain curve of a billet material. The side compression test [4] was developed to measure the flow stress of a wire rod.

To evaluate the tribological performance of the lubrication coating on billets, many testing methods have been proposed [5]. The upsetting sliding test [6] and the T-shape compression test [7] were proposed to evaluate the friction behaviour of a wire rod. The upsetting-ball ironing test [8] was developed to evaluate the sticking capability of the lubrication coating.

However, in the manufacturing shops of steel wires, the tension test is generally used to measure the flow stress of steel wires and Bowden-Leven sliding test is employed to measure the friction coefficient. The long testing time and high testing cost are the problem for the shops, and development of a simple evaluation method of the wires for cold forging is desired.

This study proposes a simple testing method to evaluate the cold forging performance of steel wires. In the proposed test, the flow stress of the wire material, the friction coefficient and the sticking capability of the lubrication coating are evaluated with a single operation by using a specimen prepared only by cutting off from the end of wire coils.

Section snippets

Design of testing method

Fig. 1 shows the basic idea of the developed testing method. Since the specimen cut off from the end of a coil has some curvature ρ, the specimen is lain down on the V-groove die with an upward convex state and then pushed into the die by the punch.

Experimental results

Experimental conditions are shown in Table 2. The wire material is a low alloy steel SCM440. The wire is finished from 11.5 mm to 11.0 mm in diameter by drawing after annealing and zinc phosphate coating. The flow curve of the wire material measured by tension test is σ = 795(ε+0.083)0.12 MPa. The friction coefficient of the lubrication coating measured by Bowden-Leven sliding test at the temperature of 50 ℃ is 0.084.

Experiments are carried out on a 1100 kN servo press. The punch load, the die

Offset method of influence of specimen curvature

Since the specimen cut off from the end of a coil has some curvature, a method to remove the influence of curvature is necessary. When the specimen is not straight, the punch load is extremely small until the specimen is straightened and thus the effect of curvature on the punch load-stroke curve can be eliminated by offsetting the measured stroke as shown in Fig. 13. Drawing a tangential line at the middle point of the rapid increase stage of the punch load, then offset the punch load curve by

Conclusions

To evaluate the forging performance of steel wires easily, a new testing method is developed and experiments with low alloy steel wires are carried out.

  • (1)

    In the developed testing method, the flow stress of the wire material and the friction coefficient of the lubrication coating are determined by a single test using a specimen cut off from the end of a wire coil.

  • (2)

    In the developed testing method, the influence of the specimen curvature can be removed easily and one set of testing tools can

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