金属热处理15

四．火焰淬火工艺

最简单的是采用氧气——乙炔火焰 简单、难以控制。

Direction of Feed

Fuel Gas Coolant

Rotating Job Direction

Movement of of Ring Shaped Burner Burner

Flame

Flame Hardened Jet of Job Zone

Coolant

(a)

(b)

Fig. 9.1 Schematic diagram illustrating two principles. (a) Progressive flame

hardening. (b) Progressive spin hardening.

五．激光加热表面淬火

激光热处理的基本原理：

为了提高吸收率需要在工程的表面涂黑。

激光热处理的特点发展和应用：

特点： 加热速度快，淬火不用冷却剂。 可以进行局部的选择性淬火。 几乎没有变形。 应用： 柴油机气缸套 弹簧片 剪刀

美国汽车工业上应用。 工艺学96页

六．电子束加热表面淬火

和激光热处理相比，各有优缺点

Fuel Gas Coolant Sprayer

七．电接触加热表面淬火

工艺学94页 图4—32

八．电解液加热表面淬火

工艺简单、生产率高、变形小，可以流水化生产，但是对于形状复杂、尺寸比较大的工件不宜采用。

9

Surface Hardening

INTRODUCTION

In this chapter, those surface hardening processes are discussed in which there is no change in the chemistry of the surface of steel component to be surface hardened. These processes are flame hardening, induction hardening, laser hardening and electron beam hardening.

9.1 FLAME HARDENING

Flame hardening is the simplest form of surface hardening heat treatment. This process consists of heating the large work-piece, such as crankshaft, axle, large gear, cam, bending roller, or any other complicated cross-section, by an oxy-acetylene or oxy-fuel blow pipe, followed by spraying of jet of water as coolant. After hardening, reheating of the parts is carried out in furnace or oil bath at about 180-200℃ for stress relieving. Such a treatment does not appreciably reduce the hardness at the surface. Hardness in flame hardened steel is due to martensitic and lower bainitic structure.

Overheating of work-piece should be avoided, otherwise, there is danger of cracking after quenching and excessive grain growth in the region just below the hardened surface. The carbon content required for flame hardening steels varies from 0.3 percent to 0.6 percent. High carbon steels can also be hardened by this process, but greater care is required to avoid cracking. Normally, case depth up to 3 mm can be achieved. A high rate of heating is essential for thin cases with proper adjustment of timing of application of flame. For good quality, strict control of heating time and fuel and oxygen consumption is essential.

There are four different methods which are used in general for flame hardening: (i) stationary, (ii) progressive, (iii) spinning, and (iv) progressive-spinning.

In the first, both burner and work-piece are stationary. Progressive hardening is carried out by using a burner combined with a waterspray, as shown in Fig. 9.1(a). In this case, the burner moves over the large stationary work-piece. This is followed by quenching. In the spinning method, the work-piece is rotated, while the burner remains stationary. After heating, the flame is removed and quenching is carried out by a water jet. In the progressive-spinning method, the burner moves over a rotating work-piece [see Fig. 9.1(b)]. In all the cases, rapid quenching is carried out after heating. There is little scaling, decarburization, or distortion in flame hardening. Since the

heating and cooling are very fast, the core remains unaffected.

Fuel Gas

Direction of Movement of Burner Flame Job

(b)

Fig. 9.1 Schematic diagram illustrating two principles. (a) Progressive flame

hardening. (b) Progressive spin hardening.

(a)

Hardened Zone

Jet of Coolant

Coolant

Rotating Job Ring Shaped Burner

Flame

Fuel Gas Coolant Sprayer

Direction of Feed

9.2 INDUCTION HARDENING

Induction hardening may be used for local surface heat treatment. Generally, it is used to surface harden crank shafts, cam shafts, gears, crank pins and axles. In this process, heating of the component is achieved by electro-magnetic induction. A conductor (coil) carries an alternating current of high frequency which is then induced in the enclosed steel part placed within the magnetic field of the coil. As a result, induction heating takes place. The heat so generated affects only the outer surface of the steel component due to skin effect.

The degree of flow of current on the outer surface of a component depends on the frequency, resistivity and permeability of the component. For a given material, The last two factors depend on temperature. The depth to which the current Penetrates and raises the temperature is given by the following relation for steel components:

In cold state (at 20℃), d20=20/√f (9.1) In hot state (at 800℃), d800=500/√f (9.2)

where d is the depth (mm) to which current flows and f is the frequency of current carried by the coil. This frequency is expressed in hertz. This relationship shows that the depth of hardening decreases with increase in frequency. In addition to direct heating of the skin by induced current, there is also some heating of the core due to conduction of heat. Hence, the overall depth of heating is greater than that given by equations (9.1) and (9.2). Accordingly, the overall depth of penetration of heat (d0, in mm) at 800℃ is given by the relation

(d0)800 = d800 + dc (9.3)

where dc (mm) is the depth of penetration of heat due to conduction and is given by the relation