GB2073255A

GB2073255A - Heat Treatment of Alloy Steel Material

Info

Publication number: GB2073255A
Application number: GB8106481A
Authority: GB
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 1980-03-05
Filing date: 1981-03-02
Publication date: 1981-10-14
Also published as: FR2477575A1; DE3108192A1; IT1170775B; IT8147932A0; GB2073255B; BE887742A; JPS56123322A; IT8147932A1; FR2477575B1

Abstract

An alloy steel material such as the fork of a motorcycle, is heated to a high temperature, e.g. 800-1200 DEG C, cooled by a first cooling step to a temperature in the range of about 400-560 DEG C, and then cooled from the foregoing temperature range in a second cooling step at a cooling rate which is greater than that in the first cooling step. The resulting alloy steel material has a good surface condition and little deformation.

Description

SPECIFICATION Heat Treatment of Alloy Steel Material This invention relates to a process for the heat treatment of an alloy steel material.

There is known a heat treatment process wherein a mild steel material or an alloy steel material is heated to a high temperature of 800-1 2000C while it is being subjected to a brazing temperature, and is cooled in a stepwise manner by a water jacket in a furnace to normal temperature. This process, however, is defective in that the resulting steel material has a lower tensile strength that the steel material before the heat treatment. For overcoming this defect, there has been proposed a heat treatment process wherein a mild steel material or an alloy steel material is heated to a high temperature for brazing, is cooled to a temperature of about 570-7200C, and thereafter is rapidly cooled by being immersed in water.This process is described in Japanese Patent Application No. 52- 86602 (Japanese Unexamined Publication No. 54-21 943). By this process, the foregoing defect can be overcome, but it often happens that a comparatively large deformation of the resultant steel material occurs, and consequently this deformation must be compensated for in a subsequent step.

Additionally, this process has the disadvantages that the cooling water decomposes to form active oxygen when it contacts the steel member at the high temperature of about 570-7200C, and that the surface of the steel material is oxidized by such oxygen. Accordingly, the bright surface thereof is lost and a subsequent pickling step for removing the oxidized material is necessary.

According to the present invention, there is provided a process for the heat treatment of an alloy steel material, which comprises heating an alloy steel material to a high temperature (with or without brazing thereof), cooling the material in a first cooling step to a temperature in the range of about 400-5600C, and further cooling the material from such a temperature range in a second cooling step at a cooling rate which is greater than in the first cooling step.

Embodying examples of this invention will now be described with reference to the accompanying drawings, which are graphs which will be explained in the description below.

The heat treatment process of the invention can be applied to various alloy steel materials comprising a carbon steel containing one or more elements such as Mn, Cr, P or the like. However, this invention will be described hereinafter, by way of example, in respect of three alloy steel materials, namely Mn-C-Fe, Mn-Cr-C-Fe and P-C-Fe.

A rear fork for a motorcycle, made of one of these three alloy steel materials, was put on its predetermined portions and brazed with copper solder or copper alloy solder or the like, and was then introduced into a heating furnace and heated to a temperature of about 800-1 2000C in a known manner.Thereafter, the material was cooled by passing water through a water jacket built into the wall of the furnace at a cooling rate which is preferably below 0.2 OC/sec. At the point in time when the temperature thereof was reduced to a temperature in the range of 5650C-4000C, a non-oxidizing gas (such as nitrogen gas, decomposed ammonia gas, a converted gas of propane or the like) was forcibly blown through the furnace by a fan whilst the foregoing water jacket cooling was continued, so that the alloy steel material was cooled at a comparatively faster cooling rate of above 0.3 OC/sec, usually at a cooling rate of about 1-1 00C/sec, in comparison with the cooling rate when subjected to the foregoing water jacket cooling alone.In an alternative embodiment, the alloy steel material was rapidly cooled from the foregoing temperature range of 565-4000C by liquid cooling, i.e. immersing the material in a cooling liquid such as water, aqueous phosphate or the like, or by applying a cooling liquid to the surface thereof in any desired manner such as by pouring, droping, flowing down, spraying or the like. Thus, the cooling of the alloy steel material is changed from a comparatively low cooling rate to a comparatively high cooling rate at the point in time when the material has been lowered to a temperature in the range of about 400-5600C. In other words, the material is subjected to a first cooling step and to a second cooling step with the temperature range of about 400-5600C being the boarder between the two steps.

The first cooling step may also be carried out by blasting a non-oxidizing gas through the furnace or by a combination of the use of the water jacket and the non-oxidizing gas. The second cooling step may be carried out, as mentioned below, by the combination of the use of the water jacket and of the gas, or by liquid cooling.

When the first cooling step is carried out by the combination of the use of the water jacket and of the non-oxidizing gas (at a cooling rate of 0.3--1 OOC/sec, for instance), the second cooling step, which is effected at a cooling rate which is faster than the cooling rate in the first cooling step, can be effected easily by liquid cooling in any desired manner as mentioned above. It is usual for a rapid cooling rate of 50-2000C/sec to be obtained by liquid cooling.

The reason why, according to this invention, the steel material of the high temperature is cooled in the first cooling step below about 5600C, i.e. below 5650C at the highest, is that this is effective for the second cooling step in which the material is subjected to the liquid cooling.

When we investigated the various oxidation conditions of the surface of the alloy steel material obtained by changing of the temperature from which the second cooling step (effected by the use of liquid) is begun we have found that, as illustrated by Fig. 1 (which shows the relationship between the oxidation amount in mg/dm2 and the temperature in OC from which the second cooling step is begun), when the liquid cooling is begun from a temperature above 5700C the surface of the resultant steel material is oxidized so that its brightness is lost and an oxidized film is formed thereon to such an extent that a subsequent pickling step is necessary, whereas when the liquid cooling is begun from below 5650C, such disadvantages do not occur and the bright surface of the steel material remains unchanged so that the subsequent pickling step is unnecessary and a good heat-treated alloy steel material can be obtained.

Another reason why the second cooling step is begun from a maximum temperature of about 5600C is that this ensures that, as illustrated by Fig. 2 (which shows the relationship between the tolerance in the width L of a rear fork in mm and the temperature in OOC from which the second cooling step is begun), the tolerance in the width L of the rear fork is within an allowable range of +2 mm.

However, when the rapid cooling is carried out from a temperature of above 570 C, this tolerance exceeds the allowable range so that a subsequent compensation step is necessary.

The reasons why the process of this invention is especially applicable to alloy steel materials and why the first cooling step is begun at a minimum temperature of 4000C and the second cooling step is carried out from this point in time will now be explained. As illustrated by Fig. 3 (which shows the relationship between the tensile strength of the product in kg/mm2 and the temperature in OC at which the first cooling step is begun), it has been found that the process of the invention is not effective for improving the mechanical strength of a carbon steel "D", and that, even in the case of alloy steel, if the temperature thereof is lowered to below 3500C in the first cooling step, the resultant product has a lower mechanical strength that the alloy steel material before the heat treatment of this invention.

Thus, the improvement in mechanical strength of the alloy steel product can be achieved only when the temperature thereof is limited to a minimum of about 4000C in the first cooling step.

The tests described with reference to Fig. 3 were carried out on an alloy steel material "A" consisting of Fe, 1.4% of Mn and 0.18% of C, an alloy steel material "B" consisting of Fe, 1.2% of Mn, 0.6% of Cr and 0.06% of C, an alloy steel material "C" consisting of Fe, 0.1% of P and 0.07% C, and a mild steel "D" consisting of Fe and 0.07% of C. Each of these raw materials were subjected to a high temperature treatment at 1 1 500C for brazing, were cooled by water jacket cooling at a cooling rate of 1 OC/sec to predetermined temperatures (usually 5500C) as shown in the Figure, and were then subjected to the second cooling step at a cooling rate of 1 500 C/sec by being immersed in water.The characteristic curves of the tensile strength of each of the products is as shown by the dotted-line curves A', B' and C'. Each of the above raw materials was subjected to the same high temperature treatment at 11 500C for brazing, was cooled to predetermined temperatures (usually about 5500C) by water jacket cooling and by a blast of non-oxidizing gas at a cooling rate of 40C/sec, and was then cooled rapidly at a cooling rate of 1 500 C/sec by being immersed in water. The characteristic curve of the tensile strength of each of the products is as shown by the solid-iine curves A, B, C and D.

As will be clear from Figure 3, it has been found that mild steel is not improved in its mechanical strength but that all of the alloy steel materials are improved in their tensile strength, as compared with the respective untreated materials, when the second cooling step is begun from a minimum temperature of near to 4000 C.

It has been also confirmed, as will be clear from Fig. 3, that, when two stages of positive cooling are carried out, all of the steel materials have a greater increase in tensile strength than when naturai cooling and positive cooling are carried out.

According to tests whose results are given in Fig. 4 (which shows the relationship between the tensile strength in kg/mm2 of the product and the cooling rate in OOC/sec), it has been confirmed that the tensile strength of the heat-treated alloy steel material is usually higher than that of the untreated material only when the second cooling step is effected at a cooling rate above 0.3 OC/sec. As a specific example, each of the above alloy steels A, B and C was cooled from 5000C by a non-oxidizing gas forcibly blown through the furnace, at various cooling rates. The resulting products were compared in tensile strength with the untreated materials, as shown in Fig. 4.

The bonding strength of the brazed portion of a brazed alloy steel material produced according to this invention was measured. Thus, a brazed test piece was repeatedly subjected to a bending stress, and the relationship between the number of applications of the bending stress before the test piece broke and the bending stress in kg!mm2, as compared to a similar test carried out in respect of a MIG welding product, is shown in Fig. 5. It will be noted from Fig. 5 that the brazing strength of the heat treated product according to this invention (line A) is good as compared to that of the MIG welding product (line B).

Although the reason why the heat treatment of this invention is especially effective for an alloy steel material is not clear, it is believed that, by the two cooling steps, especially, one or more of the alloying elements such as Mn, Cr, P and others contained in the material act to strengthen the solid solution thereof with iron, and create complex structural phases of martensite and ferrite to improve the mechanical strength of the steel and, in the case of ferrite, to improve the toughness of the steel.

As regards Mn, if it is present in an amount below 0.7%, the mechanical strengthening effect on the material resulting therefrom may be small, and if it is present in an amount about 2%, there might not be any appreciable improvement in the mechanical strength and the steel material might have reduced working properties. Thus, its presence in the foregoing range is preferable.

As regards Cr, similarly to Mn, if it is present in an amount above 1.5%, there might not be any appreciable improvement in the mechanical strength and the steel might be difficult to work. Its presence in an amount of 0.11.5% is preferable.

As regards P, if it is present in an amount below 0.05%, the improvement in the hardening effect on the steel might be decreased, and if it is present in an amount above 0.15%, the product might be brittle, and thus its presence in this range is preferable.

The alloy steel material, heat-treated by the process of this invention, may be higher in both tensile strength and yield point by about 10-60% than conventional alloy steels of high tensile strength.

In the process of the invention, if there is used, as the non-oxidizing gas, converted gas of propane, butane or the like whose dew point is + 1 0-200C, there is formed a decarburization layer of less than 0.1 mm in thickness on the surface of the alloy steel, and toughness thereof is improved.

The invention will now be illustrated by the following Examples.

Example 1 A rear fork made of an alloy steel material comprising 0.4% of C, 1.8% of Mn and the remainder Fe was set between contact portions to which it was to be brazed with copper solder. The fork was heated after being introduced into a preheating chamber of a furnace containing a non-oxidizing gas atmosphere, namely a converted gas of propane, consisting of a mixture of hydrogen (8%), nitrogen (73%), carbon monoxide (10%) and carbon dioxide (9%). Then, the fork was moved into the main heating chamber of the furnace and heated in the non-oxidizing gas atmosphere to 11 00-11 500C for 1-3 minutes, whereby the copper solder melted and filled the contact portions.Then, the fork was introduced into a water jacket cooling chamber, and was cooled by passing water at about 200C through the jacket and by blowing a non-oxidizing gas at about 200C through the chamber by the use of a fan, so that the fork was cooled to about 5500C at a cooling rate of 40 C/sec. Thereafter, the fork was introduced into a second cooling chamber, and was cooled by a fan-driven blast of a non-oxidizing gas at about 300C, at a cooling rate of 6 C/sec. The fork was taken out of the furnace when its temperature had been reduced to normal temperature.

The mechanical strength of the product thus obtained, of the untreated material and of a product obtained by a conventional furnace cooling process (namely a process in which the material was cooled from a high temperature to normal temperature in a step-like manner by using a water jacket in a furnace) were measured, and the results obtained are given in the following Table 1.

Table 1

Material Tensile strength Yield point Elongation Untreated material 65 kg/mm2 1 52 kg/mm2 18% Product of this invention 76 kg/mm2 63 kg/mm2 12% Product of conventional process 56 kg/mm2 44 kg/mm2 23% Example 2 An alloy steel material 1 mm in thickness and comprising 0.05% of C, 0.13% of P and the remainder Fe was heated to 1 100--1 1500C for 2-3 minutes in the main heating chamber of a furnace containing the converted propane gas atmosphere, and was then introduced into a water jacket cooling chamber and cooled by the water jacket (containing water at about 200C) and by a blast of the non-oxidizing gas at a cooling rate of 40C/sec. After being cooled to near 4800C, the material was introduced into a rapid cooling chamber and immersed in water so as to be cooled at a rate of 100--2000C/sec to normal temperature. Then, the material was removed.

As in Example 1, mechanical strengths were measured. The results obtained are given in the following Table 2.

Table 2

Material Tensile strength Yield point Elongation Untreated material 39 kg/mm2 29 kg/mm2 35% Product of this invention 51 kg/mm2 37 kg/mm2 30% Product of conventional process 38 kg/mm2 28 kg/mm2 37% Example 3 An alloy steel material comprising 0.06% of C, 1.30% of Mn, 1.0% of Cr and the remainder Fe was subjected to heat treatment as described in Example 2. The mechanical strength of the product was measured, and the results obtained are given in the following Table 3. The same results were obtained when, as the non-oxidizing gas, decomposed ammonia gas was used.

Table 3

Material Tensile strength Yield point Elongation Untreated material 52 kg/mm2 28 kg/mm2 32% Product of this invention 60 kg/mm2 48 kg/mm2 23% Product of conventional process 45 kg/mm2 ~ 25 kg/mm2 35% Example 4 An alloy steel material comprising 0.14% of C,1.8% of Mn and the remainder Fe was heated to 1 100--1 1500C for 1-3 minutes in the main heating chamber of a furnace under an atmosphere of converted gas of butane, and thereafter was introduced into a cooling chamber and cooled in temperature to about 5300C by a water jacket at about 300C at a cooling rate of 0.20C/sec. Thereafter the material was cooled to room temperature by a water jacket at 300C and by a blast of the foregoing non-oxidizing gas at about 2000C.

The mechanical strength of the product thus obtained was measured in comparison with an untreated alloy steel material of the same composition and a conventional product obtained by the conventional process. The results obtained are given in the following Table 4.

Table 4

Material Tensile strength Yield point Elongation Untreated material 57 kg/mm2 44 kg/mm2 23% Product of this invention 58 kg/mm2 45 kg/mm2 23% Product of conventional process 50 kg/mm2 39 kg/mm2 26% Thus, according to the invention, there is obtained a heat-treated alloy steel product which maintains its surface condition, which is improved in its mechanical strength, and which has less deformation

Claims

1. A process for the heat treatment of an alloy steel material, which comprises heating an alloy steel material to a high temperature (with or without brazing thereof), cooling the material in a first cooling step to a temperature in the range of about 400--560"C, and further cooling the material from such a temperature range in a second cooling step at a cooling rate which is greater than in the first cooling step.

2. A process as claimed in claim 1, wherein the first cooling step is carried out by water jacket cooling or by cooling with a non-oxidizing gas or by a combination thereof, and wherein the second cooling step is carried out by (a) a combination of water jacket cooling and cooling with a non-oxidizing gas or by (b) liquid cooling.

3. A process as claimed in claim 1, wherein the alloy steel material is heated to a temperature of about 800--12000C, is cooled by water jacket cooling to a temperature in the range of about 4005600C, and thereafter is further cooled by a combination of water jacket cooling and cooling with a non-oxidizing gas at a cooling rate above 0.30C/sec.

4. A process as claimed in claim 1, wherein the alloy steel material is heated to a temperature of about 800--12000C, is cooled to a temperature in the range of about 400-5600C at a cooling rate of 0.3--1 OOC/sec by water jacket cooling or by cooling with a non-oxidizing gas or by a combination thereof, and thereafter is further cooled at a cooling rate of 100--2000C/sec by being brought into contact with a cooling fluid.

5. A process as claimed in claim 1, wherein the alloy steel material is heated to a temperature of about 800--12000C, is cooled to a temperature in the range of about 400-5600C by a water jacket at a cooling rate of 1 OC/sec, and thereafter is cooled at a cooling rate of 1 500C/sec by liquid cooling.

6. A process as claimed in claim 1, wherein the alloy steel material is heated to a temperature of about 800-1 2000C, is cooled to a temperature in the range of about 400--5600C by a combination of water jacket cooling and cooling with a non-oxidizing at a cooling rate of 40C/sec, and thereafter is cooled at a cooling rate of 1 500C/sec by liquid cooling.

7. A process as claimed in claim 1, wherein the alloy steel material is heated to a temperature of 1100-1150 C, is cooled to about 5500C by a water jacket at about 20 C and by a non-oxidizing gas at about 2000C at a cooling rate of 40C/sec, and thereafter is further cooled by a water jacket at about 200C and by a non-oxidizing gas at about 30 C at a cooling rate of 6 C/sec.

8. A process as claimed in claim 1, wherein the alloy steel material is heated to a temperature of 1 1 00--1 1 500C, is cooled to about 4800C by a water jacket at about 200C and by a non-oxidizing gas at about 20 C at a cooling rate of 40C/sec, and thereafter is cooled by being immersed in liquid at a cooling rate of 100--200bC/sec to room or normal temperature.

9. A process as claimed in claim 1, wherein the alloy steel material is heated to a temperature of 1100-1150 C, is cooled to about 5300C by a water jacket at about 300C at a cooling rate of 0.2 OC/sec, and thereafter is cooled by the water jacket and by a non-oxidizing gas at about 2000C at a cooling rate of 0.30C/sec.

10. A process as claimed in any of claims 2, 4, 5, 6 and 8, wherein the liquid cooling is effected by immersing the alloy steel material in water, aqueous phosphate solution or other liquid coolant.

11. A process as claimed in any of claims 2, 4, 5, 6 and 8, wherein liquid cooling is effected by spraying or otherwise applying water, aqueous phosphate solution or other liquid coolant to the alloy steel material.

12. A process as claimed in any of claims 2, 3, 4, 6, 7, 8 and 9, wherein the cooling with a nonoxidizing gas is effected by a fan-driven stream of nitrogen, decomposed ammonia gas, converted gas of propane, butane or other non-oxidizing gas.

13. A process as claimed in any of claims 1 to 12, wherein the alloy steel material comprises 0.7-2.0% by weight of manganese, 0.020.5% by weight of carbon and the remainder iron.

14. A process as claimed in any of claims 1 to 12, wherein the alloy steel material comprises 0.72.0% by weight of manganese, 0.02-0.5% by weight of carbon, 0.1-1.5% by weight of chromium and the remainder iron.

15. A process as claimed in any of claims 1 to 12, wherein the alloy steel material comprises 0.05-0.1 5% by weight of phosphorus, 0.02-0.5% by weight of carbon and the remainder iron.

16. A process as claimed in any of claims 1 to 15, wherein the alloy steel material is in the form of a fork for a motorcycle.

17. A process as claimed in claim 1, substantially as described with reference to the Figures.

18. A process as claimed in claim 1, substantially as described in any of the Examples.

19. An alloy steel material which has been heat treated by a process as claimed in any of claims 1 to 18.