WO2000022177A1

WO2000022177A1 - Heat treatment of wearable working tool

Info

Publication number: WO2000022177A1
Application number: PCT/GB1999/003111
Authority: WO
Inventors: Furre Arnold
Original assignee: Kverneland Klepp As
Priority date: 1998-10-14
Filing date: 1999-09-17
Publication date: 2000-04-20
Also published as: EP1038040A1; PL341048A1; NO20002641D0; NO20002641L; GB9822392D0; AU6101599A

Abstract

A method of heat treatment of a wearable working tool (20, 36), particularly a plough share (20), having a leading edge region (22) which is subject to wear during ploughing, and a mounting region (23) having spaced mounting holes (24) by which the share can be mounted on a saddle, in which the treatment comprises the steps of: heating the entire body of the share to a sufficient extent, followed by rapid cooling, so as to harden the share and to provide a required level of wear resistance to the leading edge region (22); and subsequently carrying out localised tempering of at least the areas surrounding the mounting holes (24) in the mounting region (23), on each side of the share, in order to remove at least some of any embrittlement of such areas caused by the initial wear-resistant hardening step, and thereby to improve the resistance of such areas to possible stress fracturing under load.

Description

HEAT TREATMENT OF WEARABLE WORKING TOOL

This invention relates to the heat treatment of a wearable working tool, such as a ploughshare, with a view to providing improved strength in the mounting of the tool together with improved wear resistance of wear regions of the tool.

In use, soil or ground-working tools such as ploughshares are exposed to heavy loads and wear problems, and are a "consumable item" which require regular replacement whenever (a) an unacceptable level of wear has taken place or (b) stress cracks or other damage appears on the share (usually in the region of mounting holes by which the share is mounted on the plough body saddle) .

A ploughshare is usually made of hardened steel plate, and has a leading edge which is exposed to substantial abrasive action as it moves through the soil which is being ploughed, and over a period of time the edge wears away thereby reducing its effectiveness. The share is usually mounted on the saddle via a mounting region of the share having a series of mounting holes which are spaced laterally inwardly of the leading edge, and as wear of the leading edge occurs, the width of the share reduces in a direction towards the mounting holes.

The leading edge region of the share i.e. the leading edge plus the adjoining material through which wear takes place, must be hardened throughout the share so that it can withstand the abrasive forces to which it is subjected, and therefore it is usual to apply a suitable heat treatment to the entire share so as to provide a sufficient degree of hardness throughout the share in the leading edge region. Existing heat treatments comprise heating of the share throughout its thickness to a substantially uniform temperature followed by rapid cooling of the entire share by quenching.

However, the share also has a mounting region, which comprises an edge region opposite to the leading edge and which includes a series of mounting holes through which suitable fasteners can be taken in order to mount the share on the saddle. The fasteners are subjected to substantial loads in service, both by application of downward loads from the plough frame, and also by application of turning moments to the share, both of which result in application of substantial, and variable shear forces between the fasteners and the walls of the holes in which they are mounted. These are concentrated forces giving rise to variable high pressure contact between the fasteners and the walls of the mounting holes, and which are liable to result in generation of stress cracks in the surface of the share in the region of the mounting holes over a period of time.

Thus, when the share and saddle are put together and fastened with bolts and nuts, then there is added stresses and there comes into being a complicated stress pattern in the share. There are contact stresses between the parts, especially around the mounting holes and since the surfaces of the share and the saddle are not machined, there can also be bending stresses in the parts when they are forced together by tightening of the bolts and nuts. These are static stresses which are always present, and which can result in cracking of the share. In addition, in operation, there are dynamic stresses, and which can give rise not only to shear forces between the fasteners and the walls of the holes, but also bending forces giving rise to additional bending stresses in the share.

This problem is made worse by current methods of heat treatment of shares (to improve the hardness of the surface in the leading edge region) , since heat treatment for improving wear resistance (to increase surface hardness) necessarily results in embrittlement of the share. This embrittlement applies throughout the share with the existing heat treatment method, and makes early stress fracture failure in the mounting region more likely.

The generation of stress cracking in wear parts of working tools can have a number of causes. One source of the cracking problem can arise due to so-called "hydrogen embrittlement" . There is always present some hydrogen gas in steels, which can have an adverse effect on the mechanical properties of the metal part, and particularly because of the possibility of generation of hydrogen-induced cracking. It is technically possible to seek to prevent such cracking occurring by use of special coatings which limit hydrogen gas entry into the metal, but such coatings are easily damaged, and soon wear away during service. Furthermore, the provision of special coatings involves additional constituent materials, and further process steps.

It is also a fact that, as the requirements for yield strength increase, the chances of hydrogen-induced cracking also become greatly increased. Some of the most serious problems associated with hydrogen damage occur as delayed failures under constant load in service.

Therefore, with the existing heat treatment procedure (heating followed by rapid cooling by quenching) , it is necessary to make some compromise between (1) production of a very hard share with excellent wear resistance (for the wear region) but poor mechanical strength in the mounting region due to hydrogen or other embrittlement and (2) a share with satisfactory mechanical strength (for the mounting region) but lower hardness so that the wear region has a reduced working life.

The invention therefore seeks, in one aspect, to provide an improved heat 'treatment for shares, in which enhanced wear resistance can be obtained in the wear region and enhanced resistance to stress fractures in the mounting region.

According to a first aspect of the invention there is provided a method of heat treatment of a share for a plough, said share having a leading edge region which is subject to wear during ploughing, and a mounting region having spaced mounting holes by which the share can be mounted on a saddle, in which the treatment includes the steps of: heating the entire body of the share, followed by quenching, in order to harden the entire body of the share and provide a required level of wear resistance to said leading edge region; and, subsequently carrying out localised tempering of at least the areas surrounding the mounting holes in the mounting region, on each side of the share, in order to remove at least some of any embrittlement of such areas caused by the initial wear-resistant hardening step, and thereby to improve the resistance of such areas to possible stress fracturing under load.

The means by which the heating of the entire body of the share is carried out (in the initial wear-resistant heating step) can be achieved in a number of ways. For example, it can be achieved by moving the share body through a tunnel oven so that the entire body is heated. The heated share body is then subjected to rapid cooling e.g. in a quenching bath so as to give a required level of hardness throughout the share body.

The localised tempering is preferably carried out by an inductive heating technique, in which relative movement takes place between the mounting region of the share and a number of inductive heating heads which are spaced apart to define a gap therebetween in which the mounting region can be received.

Conveniently, the heating heads are fixed and are arranged in two lines of heads, with the lines being separated from each other to define the gap in which the mounting region of the share is received.

The share is preferably manipulated by a robot arm which removes the share from a quenching bath and transfers the share to a tempering station which is provided with the arrangement of lines of heating heads.

Advantageously, the method of the invention is applied on a mass production basis, in which batches of working tools (share bodies) are passed along a heat treatment production line which preferably comprises:

(a) a heating tunnel or oven which receives batches of forged working tools (ploughshares) to be heat treated;

(b) a quenching tank or bath to which the batches are transferred by a robot arm;

(c) a dwelling region in which the quenched parts are maintained for a predetermined period, being transferred thereto also by the robot arm; (d) a tempering station having two lines of induction heating heads, and defining a gap through which the tools are moved individually by the robot arm, to carry out localised tempering of the mounting region; and,

(e) a cooling tunnel or other cooling arrangement to which the locally tempered tools are transferred by the robot arm.

In carrying out a heat treatment method according to the invention on a ploughshare, a heat treated share can be produced having improved wear resistance by reason of the hardness applied to the leading edge region and without any appreciable adverse effect on the mechanical strength and stress-crack resistance of the mounting region.

Preferred examples of methods of carrying out heat treatment of a wearable working tool according to the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is an illustration of a share mounted on a saddle of plough, showing how stress cracks can be generated in the region of the mounting holes of the mounting region by which the share is mounted on the saddle;

Figure 2 illustrates a replacement share in the right hand view, and a worn-out share in the left hand view;

Figure 3 shows the worn-out share superposed on the replacement share, to show the level of wear which can occur in the leading edge region over a period of time;

Figure 4 is a plan view of a heat treated share having undergone a heat treatment method according to the invention; Figure 5 is a schematic illustration of a series of stages in a batch type of heat treatment production line for carrying out a method according to the invention; and,

Figure 6 shows a further example of a wearable working tool to which the heat treatment method of the invention may be applied.

Referring first to Figure 1 , this shows a ploughshare 10 mounted on a saddle 11 of a plough, in which the share 10 has a leading edge region 12 which is exposed to abrasive wearing action during ploughing, and an opposed mounting region 13 having a series of spaced mounting holes 14 through which fasteners are taken in order to mount the share 10 on the saddle 11.

The share 10 is made of hardened steel plate, and the leading edge region 12 and the mounting region 13 comprise opposed edge regions of the plate.

Conventional heat treatment techniques applied to the share 10 comprise overall heat treatment of the share, followed by quenching, to apply a sufficient level of hardness, so that the leading edge region 12 has wear resistance to withstand the abrasive action to which it is exposed during ploughing. However, by reason of this treatment applied to the share, while it gives necessary hardness for the wear region, by being applied also to the entire share results in reduction in strength (and resistance to stress fractures) in the mounting region 13. This is believed to be because of embrittlement by reason of the hardness treatment, and which therefore reduces the working life of the share, by reduction of the strength in the mounting region. The mounting holes in the mounting region 13 receive fasteners which secure the share 10 to the saddle 11 , and substantial high pressure contact takes place between the fasteners and the walls of the mounting holes. This is by reason of the downward vertical loads applied to the fasteners via the plough frame, as well as turning moments applied to the ploughshare during ploughing. Stress fractures generated in the mounting region 13 are apparent from Figure 1.

Referring now to Figure 2, the right hand view shows a new replacement and unworn ploughshare 10a, having an unworn leading edge region 12a and an opposed mounting region 13a having a series of mounting holes 14a.

The left hand view shows the share 10a after it has undergone a substantial degree of wear, whereby the width of the share 10a (measured between the mounting region 13a and the leading edge region 12a) is substantially reduced, such that the leading edge region 12a has worn away, and the new position of the leading edge is now shown by reference 12b.

Figure 3 shows the worn share superposed on the unworn replacement share, and shows the extent of wear which can take place in service.

Referring now to Figure 4, this is a plan view illustration of a heat treated share which has been subjected to a heat treatment method according to the invention. The share shown in Figure 4 is designated generally by reference 20 and will usually be made of suitable steel alloy having constituent proportions determined according to requirements. The share is formed into required shape by forging from steel plate, and typically this involves a number of separate forging operations, in which a hot metal billet is cropped to shape, and also undergoes lateral deformation during forging, in order to thicken part of the metal plate where additional strength is required i.e. in the mounting region.

The share 20 has a leading edge region 22 of maximum hardness, and an opposed mounting region 23 having mounting holes 24 by means of which the share 20 can be mounted on a saddle of a plough (not shown) . Fasteners 25 are taken through the holes 24 in order to mount the mounting region 23 of the share 20 on the plough saddle.

The mounting region 23 is heat treated to have medium hardness, and there is a transition zone 26 between the mounting region 23 of medium surface hardness and the leading edge region or wear region 22 which has maximum hardness .

The maximum hardness is applied to the wear region 22, to give it necessary resistance to the abrasive action to which it will be subject in service, and while such heat treatment may result in some loss of strength and embrittlement, this is not a problem to the overall strength of the share 20, because the wear region 22 is mainly subjected to wearing forces, rather than localised highly stressed regions, which can apply in service to the mounting region 23. It is for this reason that the mounting region 23 receives a subsequent localised heat treatment whereby the hardness is not as great as the wear region 22, so that the mounting region 23 can have improved mechanical strength, and a greater resistance to^' possible generation of stress cracks . Bearing in mind the high pressure contact which will arise in service between the fasteners 25 and the walls of the holes 24, it is important that the mounting region 23 should have necessary strength and ability to resist stress cracks being generated in service.

The method of heat treatment of the ploughshare 20 can take a number of different ways, within the scope of the claimed invention. In a first example, the entire body of the share is heated e.g. in a heating oven or tunnel, so that the share 20 is heated throughout to a sufficient extent (followed by rapid cooling) so as to harden the share and to provide a required level of wear resistance in the leading edge region 22. Typically, this can involve heating of the share 20 to a temperature in the range 830 to 900°C, and followed by quenching to reduce the temperature of the heated share rapidly to room temperature so that the internal structure of the share (austenitic) after heating is transformed to martensitic form.

Thereafter, localised tempering is applied to at least the areas surrounding the mounting holes 24 in the mounting region 23, in order to remove at least some of any embrittlement which may be applied to such areas caused by the initial wear-resistant hardening step (heating and rapid cooling) , and thereby to improve the resistance of such areas to possible stress fracturing under load. Typical tempering temperatures in the range 160 to 680°C may be applied, and preferably in the range 400 to 600°C.

The initial wear-resistant hardening step may be carried out in a number of ways. First of all, it can be achieved by heating the entire share body, as described earlier, followed by rapid cooling e.g. in a quenching bath, so as to give an initial level of hardness throughout the share. This is then followed by localised tempering of the mounting region, at least around the mounting holes, and which results in reduced hardness of the mounting region, but which will be of little concern in practice because the mounting region is not exposed to the same level of abrasive action as the wearing region. However, the tempering action in the mounting region removes any embrittlement generated as a result of the initial hardening step.

An alternative method of carrying out the two stage hardening and tempering treatment is possible, in which the share is first hardened by heating followed by quenching as described above. This is then followed by tempering the entire share, followed by hardening (heating and quenching) only the wearing region again. This alternative procedure will give generally the same properties to the share as the first described method.

A still further alternative is to harden (heat plus quench) only the wearing region. The area around the mounting holes, in the mounting region, will then be in the state as originally (following completion of the forging step), and will therefore be weaker, but the reduction in strength compared to the hardened parts of the share can, if necessary, be compensated by providing a thicker share plate in the mounting region.

The second and third variations to the treatment method described above are intended to come within the scope of this invention.

Referring now to Figure 5, this is a schematic illustration of a heat treatment production line for use in heat treating batch supplies of forged parts . The production line comprises a first heat / hardening stage 30, which comprises a heating tunnel or oven, preferably gas heated, through which batches of forged parts are conveyed in order to apply overall heating to the entire share bodies, to a sufficiently high temperature, and for sufficient duration so that, upon transfer (preferably by a robot arm) to a subsequent quenching tank 31 , required surface hardness can be achieved for the exposed working surfaces of the shares. The heating and quenching stages result in conversion of the internal microstructure of the steel alloy from austenitic to martensitic form. After quenching in the tank 31, the robot arm then transfers the quenched parts preferably to a dwelling tank or tank portion 32, for a predetermined time period, before transfer of each share plate individually by the robot arm to a tempering station 33.

The tempering station 33 comprises two rows of inductive heating heads 34 which define between the rows a gap through which the share plate can be received and relatively moved, so that heat is induced in the mounting region of the share. The robot arm is programmed to move the mounting region, and particularly the region surrounding the mounting holes, for a sufficient time period that localised tempering takes place. This reduces the surface hardness of the heat treated and quenched share, in the mounting region only, which thereby increases the strength and resistance to stress-generated cracks in service.

Preferably, the robot arm moves the share plate through fixed inductive heating heads 34, but evidently a reverse arrangement will be possible within the scope of the invention, provided that it permits relative movement between the tempered share and the inductive heating heads.

After completion of the local tempering action, the shares are then transferred by the robot arm to a cooling station 35, comprising a cooling tunnel through which the shares are conveyed in order to receive a required cooling action, prior to discharge to a storage bin or the like (not shown) .

The heat treatment method according to the invention has been described in relation to preferred application to heat treatment of ploughshares for mounting on the saddle of a plough body, but it should be understood that this is merely one example of a wearable working tool to which the invention may be applied. Other examples of soil or ground- working tools to which the invention may be applied include a digging or working tooth, (point or tine) , for application to a working edge of an excavator bucket or the like. Figure 6 shows schematically a digging tooth 36, which is reversible, in the sense that it has opposed working ends 37, and when one of the working ends 37 becomes unduly worn, the tooth 36 is reversed on its mounting, so as to bring the previously unused working tip into a working position. The tooth 36 therefore has a central mounting hole 38, by means of which it can be reversibly mounted on the leading edge of an excavator bucket or the like, by use of suitable fastener taken through the hole 38.

The tooth 36 therefore can be subjected to a hardening treatment which is generally similar to that described above in relation to treatment of a ploughshare i.e. initial heat treatment and quenching so as to apply a required level of hardness and wear resistance to working or leading edge regions of the tooth, followed by localised tempering of at least the areas surrounding the mounting hole or holes in the mounting region of the tooth, in order to remove at least some of any embrittlement of such area(s) caused by the initial wear-resistant hardening step, and thereby to improve the resistance of such area(s) to possible stress fracturing under load.

Claims

1. A method of heat treatment of a share (20) for a plough, said share having a leading edge region (22) which is subject to wear during ploughing, and a mounting region (23) having spaced mounting holes (24) by which the share can be mounted on a saddle, in which the treatment comprises the steps of: heating the entire body of the share to a sufficient extent, followed by rapid cooling, so as to harden the share and to provide a required level of wear resistance to said leading edge region (22) : and, subsequently carrying out localised tempering of at least the areas surrounding the mounting holes (24) in the mounting region (23) , on each side of the share, in order to remove at least some of any embrittlement of such areas caused by the initial wear-resistant hardening step, and thereby to improve the resistance of such areas to possible stress fracturing under load.

2. A method according to claim 1, in which the initial wear-resistant step is carried out by moving the share body through a heating tunnel or oven so that the entire body is heated to a sufficient extent that, upon quenching, required hardness of the share can be achieved.

3. A method according to claim 1 or 2 , in which the localised tempering is carried out by an inductive heating technique, in which relative movement takes place between the mounting region (23) of the share and a number of inductive heating heads (34) which are spaced apart to define a gap therebetween in which the mounting region can be received.

4. A method according to claim 3, in which the heating heads (34) are fixed and are arranged in two rows of heads, with the rows being separated from each other to define the gap in which the mounting region of the share is received.

5. A method according to claim 4, in which the share is manipulated by a robot arm which removes the share from a quenching bath (31, 32) and transfers the share to a tempering station (33) which is provided with the arrangement of rows of heating heads (34) .

6. A method according to claim 1 and applied to mass production of heat treated share bodies, in which batches of share bodies are passed along a heat treatment production line which comprises:

(a) a heating tunnel or oven which receives batches of forged share bodies to be heat treated;

(c) a dwelling region in which the quenched parts are maintained for a predetermined period, being transferred thereto also by the robot arm;

(d) a tempering station having two lines of induction heating heads, and defining a gap through which the share bodies are moved individually by the robot arm, to carry out localised tempering of the mounting region; and,

(e) a cooling tunnel or other cooling arrangement to which the locally tempered shares are transferred by the robot arm.

7. A method according to any one of claims 1 to 6, in which the initial heating of the share body is to a temperature in the range 830 to 900°C, and the subsequent localised tempering after quenching is to a temperature in the range 160 to 680°C.

8. A method of heat treatment of a wearable working tool (20, 36), said tool having a leading edge region (22, 37) which is subject to wear, and a mounting region (23, 38) having a mounting hole or holes (24, 38) by which the tool can be mounted, in which the treatment comprises the steps of: heating the entire body of the tool (20, 36) to a sufficient extent, followed by rapid cooling, so as to harden the tool and to provide a required level of wear resistance to said leading edge region (22, 37); and, subsequently carrying out localised tempering of at least the area surrounding the mounting hole (24, 38) in the mounting region, on each side of the tool, in order to remove at least some of any embrittlement of such area caused by the initial wear-resistant hardening step, and thereby to improve the resistance of such area to possible stress fracturing under load.

9. A method of heat treatment of a share (20) for a plough, said share having a leading edge region (22) which is subject to wear during ploughing, and a mounting region (23) having spaced mounting holes (24) by which the share can be mounted on a saddle, in which the treatment comprises the steps of: heating the entire body of the share to a sufficient extent, followed by rapid cooling, so as to harden the share and to provide a required level of wear resistance to said leading edge region; subsequently carrying out tempering of the share, in order to remove at least some of any embrittlement caused by the initial wear-resistant hardening step; and, heating the wear region of the share, followed by rapid cooling, to provide required level of hardness of the leading edge region.

10. A method of heat treatment of a share (20) for a plough, said share having a leading edge region (22) which is subject to wear during ploughing, and a mounting region (23) having spaced mounting holes (24) by which the share can be mounted on a saddle, in which the treatment comprises the steps of: providing increased thickness to the share plate in the mounting region; and, heating the leading edge region (22), followed by rapid cooling, so as to harden the share and provide a required level of wear resistance to the leading edge region.