JPS6130623A - Heat treatment for high strength and high ductility spheroidal graphite cast iron - Google Patents

Abstract

PURPOSE:To obtain the titled cast iron having uniform quality without deviation in quality such as material characteristic, by jet cooling spheroidal graphite cast iron from austenitic range, furthermore applying isothermal transformation thereto using fluidized layer to bainitize the matrix structure. CONSTITUTION:Spheroidal graphite cast iron (FCD) is cooled rapidly to 200- 300 deg.C temp. higher than Mp poing from austenite range higher than A1 transformation temp. to suppress pearlite transformation. Thereafter, FCD is cooled by using mixed spray flow of water and high pressure air. Next, said cooled FCD is successively held isothermally by fluidized layer. Namely, by this invention, since spray cooling from austenite range and austemper treatment by fluidized layer are performed, high strength, high toughness FCD without deviation in quality such as material characteristic is obtained. Since conventional salt bath method is not used in the method, FCD can be treated without accompanying environmental pollution.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for heat treating high-strength, high-toughness spheroidal graphite cast iron.

Conventional technology As a method of making spheroidal graphite cast iron (FCD according to the JIS standard, hereinafter simply referred to as FCD) f into a material with even higher strength and toughness, there is a method of converting the matrix structure into bainite through austempering treatment. This is so-called bainitic spheroidal graphite cast iron.

Problems to be Solved by the Invention The common method for performing the above austempering treatment is the salt bath method in which quenching is performed in a salt bath of nitrate, etc. However, quenching in a salt bath causes variations in quality such as material properties. There are also many problems in terms of pollution.

Means and Effects for Solving the Problems The present invention was conceived in view of the above-mentioned problems, and uses jet quenching and temperature maintenance using a fluidized bed as a method of quenching and austempering without using a salt bath. It is a high-quality product that uses a spray flow of a mixture of water and high-pressure air from the austenite region with temperatures above FCD I-4 and WI4 to achieve temperatures even higher than the Ah point (200 to 350℃) at a cooling rate that does not cause pearlite transformation. ) and further performs isothermal transformation using a fluidized bed to produce high-strength, high-toughness spheroidal graphite cast iron with a bainite base.

Example FCD Fig. 1 shows the heat treatment method for converting into bainite, and Fig. 2 shows the C(,'T curve and the cooling curve of austempering treatment. In Fig. 1, 0) indicates the ferrite annealing. , ■Austenitization, O) quenching, ■austempering treatment (constant temperature maintenance 1, ■air cooling, and ■finishing).

As can be seen from Figure 1, in order to convert /''(1'7) into bainite, 01 ferrite annealing to suppress deformation after heat treatment, el uniform austenite annealing at about 920°C, 0) quenching, 0250 Austempering treatment at ~350℃ (constant temperature maintenance), 0) air cooling, but CCT in Figure 2
As can be seen from the curve, in order to obtain a stable bainite structure of good quality, it is necessary to 1) rapidly cool the austenite region above the AI transformation temperature to a temperature above the Mst point to prevent pearlite transformation; , 11) It is important that the austempering treatment (maintaining constant temperature) be performed stably.

In the method of the present invention, a spray quenching method was used for cooling from the temperature of the austenite region, and a fluidized bed was used for constant temperature maintenance.

Spray quenching is a method of cooling a workpiece by spraying a high-speed fine spray made of water and high-pressure air onto the workpiece, and has the following characteristics.

1) By controlling the amount of water, a wide range of cooling capabilities can be easily obtained from water spray cooling to air cooling.

11) Compared to water quenching or salt bath quenching, it has better uniform cooling properties and less quenching distortion.

l111 Safe and non-polluting.

FIG. 3 shows the relationship between the amount of water in spray quenching and the cooling curve. In Jin's example, the quenching temperature of the workpiece was 850° C., and the water pressure was I, 05 u. The size of tfc work is 20
It is a flat plate with tan x 30 w x 6 Qmm.

When austempering is performed, the level of machining is reduced to level 1 compared to before the treatment, so after treatment, only a small amount of finishing work must be done, but it is possible to use a spray that causes less distortion due to quenching. Quenching can be said to be the most suitable cooling method for austempering.

FIG. 4 is a diagram showing the hardness of the tooth root and pitch line portion when the gear is hardened by 91M, and it can be seen that the hardening depth is improved compared to oil hardening. In addition, the residual stress at the tooth bottom is -4υ in oil quenching, but it is -36Yi in spray cooling, which means that the compressive stress is high and an improvement in fatigue strength can be expected.

Following the spray hardening as described above, constant temperature maintenance is performed using a fluidized bed. As mentioned above, in constant temperature maintenance, it is important to maintain the temperature stably.

The characteristics of the fluidized bed are: 1) It has excellent heat uniformity as shown in Figure 5. 11) As shown in Figure 6, the movement of particles is active and the temperature rises and cools quickly. In the example shown in FIG. 6, a cylindrical steel material with a diameter of 150 m is used, and the cooling curve at the center of this material is shown.

Figures 7 and 8 are for austempered products) F
The mechanical properties of CD are shown. As can be seen from this figure, the strength of the cast steel I 5USrun-2 is much higher than that of the As-CAST product of bainite FCD d FCD.
Compared to 1, it has the same or higher strength.

Effects of the Invention According to the method of the present invention, since the austenite region is jet-cooled and the TFC austempering treatment is performed in a fluidized bed, uniform quality with no variation in material properties etc. can be obtained. and can be treated without causing pollution.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the temperature per hour of heat treatment of F(, 'D), Fig. 2 is a diagram showing the cooling curve of austempering treatment, Fig. 83 is a diagram showing the cooling curve of 1jn cooling, Fig. Figure 4 is a diagram showing the hardness distribution of gears, Figure 5 is a temperature distribution diagram in the fluidized bed furnace, Figure 6 is a diagram showing the cooling curve at the center of the cylindrical steel material, and Figure 7 is a diagram showing the mechanical properties. In the diagram, Figure 7 (, 1 indicates tensile strength, Figure 7 (gradient indicates hardness, respectively). Figure am-CAST FCD 01-Ichipo C'D
'MFLυ ''-14F Kaburagu

Claims (1)

[Claims] Spheroidal graphite cast iron is cooled from the austenite region above the A_1 transformation temperature to a temperature higher than the Ms point using a spray flow of a mixture of water and high-pressure air at a cooling rate that does not cause pearlite transformation, and then isothermal transformation is carried out using a fluidized bed. A heat treatment method for high-strength, high-toughness spheroidal graphite cast iron, characterized in that the matrix structure is changed to bainite by the heat treatment.