WO1998038346A1

WO1998038346A1 - Method and installation for tempering metallic strips

Info

Publication number: WO1998038346A1
Application number: PCT/GB1998/000617
Authority: WO
Inventors: Andrew Peter Lee
Original assignee: The L.S. Starrett Company
Priority date: 1997-02-26
Filing date: 1998-02-26
Publication date: 1998-09-03
Also published as: AU6305198A; GB9703957D0; EP0961840A1

Abstract

The present invention relates to a tempering system and method for use in the heat treatment of elongate strip material of the type used in for example band saw blades. The apparatus (1) comprises an elongate furnace (2) for heating a continuous length of bi-metal strip (4) held by and transported by a series of rollers (6). At each end (8, 10) of the furnace (2) are quenching zones (12, 14) for cooling rapidly the strip metal (4) as it exits each side of the furnace (2). The rollers are arranged so that a strip of bi-metal held on a supply coil (15) passes through the furnace (2) a first time, through a first quenching zone (12) at one side of the furnace (2) and out of the furnace and into a second quenching zone (14); around a further return roller (17) and back through the furnace (2) for at least a third time. It is then quenched by the quenching zone before being coiled up on a finish coil. The apparatus (1) may be provided with a hardening heating unit (19) placed between the supply coil (15) and the first quenching zone (12) of the system.

Description

METHOD AND INSTALLATION FOR TEMPERING METALLIC STRIPS

The present invention relates to a tempering system and apparatus for use in the heat treatment of metal materials and in particular elongate strip metal of the type used in for example band saw blades .

The conventional and known method of heat treating saw blades involves firstly hardening the material at very high temperatures e.g. 1200°C followed by quenching and then placing in a tempering furnace and soaking at a temperature below the critical temperature e.g. 550°C for a number typically three of predetermined, e.g. two hour periods, each soak being followed by quenching so as to increase the hardness by so-called secondary hardening.

The conventional method is essentially a batch process and involves the uncoiling of a typically 500 feet (152.4m) long coil of band saw material and passing this through the first hardening or austenitising process in a continuous flow process, and then coiling up again into a roll. The coils are then placed in the tempering furnace for tempering and it will be appreciated that in a large roll of material the outside of the roll will reach the desired tempering temperature more quickly than the core portions resulting in outer portions receiving a longer period at the tempering temperature and thereby uneven tempering of the material and inconsistent results. To seek to overcome this problem the winding on the coil is reversed - "coil reversing" .

Conventional methods involving "coil reversing" between tempering operations can give rise to chipping of teeth on the band saw. It is an object of the present invention to avoid or minimize one or more of the foregoing disadvantages .

It has now been found that at least equivalent, if not superior, properties and performance of high speed steel (HSS) can be achieved by changing from typically three two hour "soaking" periods at 550/560°C to a similar number of much shorter cycles e.g. 5 mins at somewhat elevated tempering temperatures which are somewhat higher than conventional tempering temperatures e.g. in the range of 550- 600 °C, with each heating stage being followed by quenching, in a continuous process tempering of elongate strip material such as band saw blades .

Thus the present invention provides a tempering system for use in the heat treatment of elongate strip metal, which system comprises material heating means formed and arranged for heating material to be treated to a tempering temperature in a heating zone thereof; material quenching means for cooling material heated to said tempering temperature; and material transport means for conveying substantially continuously a substantially continuous elongate length of said material to be heat treated sequentially through said heating zone and said material quenching means, so that said material is heated to said tempering temperature for a predetermined tempering period as it passes through said heating means and is substantially cooled and hardened upon passing through said quenching means.

Thus with a tempering system according to the present invention a very long length of elongate strip material may be rapidly and safely tempered in a substantially continuous "in-line" process. Advantageously the tempering system of the invention is formed and arranged so as to provide at least two tempering cycles, for example by providing a series of successive tempering heating zones and quenching means or by recycling the strip one or more times through a single tempering heating zone and quenching means .

Preferably said tempering system is coupled in-line with a hardening system having a material hardening heating means for heating said material to a hardening temperature; said hardening heating means and said tempering system being spaced apart by a quenching zone; whereby said material to be treated passes from said hardening heating means via said quenching zone to said tempering heating zone of said tempering system.

The hardening heating means is formed and arranged to heat strip metal passing therethrough to a substantially elevated temperature above the critical temperature of the metal so as to transform the metallurgy of the metal to austenite.

Preferably said tempering temperature is any temperature below the critical temperature, that is the temperature at which there is a change in the metallurgical structure so that the retained austenite in the material is transformed to martensite. Preferably the tempering temperature for use with the system according to the invention is in the range of 550° to 650°C, desirably 580-610°C. The hardening temperature is preferably in the range of 1100 to 1200°C desirably 1150-1190°C. Desirably said material transport means of said tempering system is formed and arranged with a material transport means of said hardening means so that material passes through said hardening means first and once only before entering the tempering system.

Preferably metal material, having been first heated in said hardening heating means and quenched in said quenching zone between said hardening heating means and said material heating means passes through said tempering heating zone at least twice, desirably three though it may be passed up to six times depending on material mass and size and is quenched after each successive pass therethrough.

Preferably said tempering heating zone of said material heating means comprises a single elongate furnace means provided, at each end thereof, with a quenching zone of said quenching means and material transport return means for recycling material exiting a said end of said tempering heating zone and quenched in said quenching zone, back into the tempering heating zone of said tempering heating means, whereby said successive passes through said tempering heating zone and exposure to said tempering temperature and rapid cooling by quenching in said quenching zone cause said material to exhibit secondary or precipitation hardening.

Preferably said material transport means comprises at least one of material pull through apparatus; turntable; wind up system; coiling and uncoiling apparatus and a plurality of roller guides formed and arranged for guiding metal material to be treated held in an untreated coiled form through said respective hardening, tempering and quenching zones and into a second treated coiled form. Desirably said material transport means and said heating means and quenching means is formed and arranged so that two lengths of elongate strip metal may be processed more or less continuously and simultaneously in side-by-side, parallel relation.

Generally the tempering furnace means has a length of approximately 40 to 50 feet (12-15m) though longer or shorter furnace means may be used depending inter alia on the strip transport speeds used and tempering heating zone residence requirements.

Said tempering heating zone of said material heating means is preferably formed and arranged to heat material to be treated up to a temperature in the range of 550 to 650°C. Said material transportation means is preferably formed and arranged for exposing said material to be treated to said tempering temperature in said tempering heating zone of said material heating means for a period of time of from four to thirty minutes, desirably five to 15 minutes depending on material mass and size, during the or each passage therethrough.

Preferably said quenching means are formed and arranged for cooling rapidly material exiting said tempering zone to a generally ambient temperature. Any suitable type of quenching means known in the general state of the art, such as water, oil or blasted chilled air, may be used.

In another respect the present invention provides a method of tempering continuous extended lengths of strip metal comprising the steps of subjecting said strip metal to at least one, preferably two, tempering cycle, each said tempering cycle comprising feeding said strip metal through a tempering furnace in a substantially extended form, for a short period of time at an elevated tempering temperature, sufficient for tempering said strip metal without softening thereof, and then through a quenching means.

In a further respect the present invention provides a method of heat treating elongate strip metal, so as to increase its hardness which method comprises the steps of : providing a tempering system according to the invention; passing said material more or less continuously through said material heating means and said quenching means sequentially by said material transport means so that said material is heated to a tempering temperature for a more or less predetermined period of time and cooled more or less rapidly.

Preferably said method includes an additional first step prior to heating to said tempering temperature of heating to a substantially elevated hardening temperature and quenching.

Further preferred features and advantages of the present invention will appear from the following detailed description given by way of example of a preferred embodiment illustrated with reference to the accompanying drawings in which:- Fig. 1 is a schematic diagram showing a band saw strip hardening and tempering system according to the present invention;

Fig. 2 is a graph showing the temperature profile for a length of material tempered by the tempering system shown schematically in Fig. 1;

Fig. 3 shows a series of photomicrographs illustrating the microstructure of band saw blade strip treated by the system; and Fig. 4 and 5 are graphs showing comparatively the performance data of band saw blades treated by the system.

A hardening and tempering system and apparatus for use in the heat treatment of band saw blades is shown in Fig. l and referred to by reference number 1. The apparatus 1 comprises a furnace 2 for heating a continuous length of bi-metal strip 4 (27mm x 0.9mm M42 metal) held by and transported by a series of rollers 6. At each end 8, 10 of the furnace 2 are quenching zones 12, 14 for cooling rapidly the strip metal 4 as it exits each side of the furnace 2.

The rollers are arranged so that a strip of elongate bi-metal strip 4 held on a supply coil 15 passes through the furnace 2 a first time, indicated by reference Tl, through the left hand quenching zone 12 at the left hand side 8 of the furnace 2, around a return roller 16 and back into the furnace (indicated by T2) and out of the furnace 2 into a right hand quenching zone 14; around a further return roller 17 and back through the furnace 2 for a third time (indicated by T3) and is then quenched by the left hand quenching zone 12 before being coiled up on a finish coil 18.

The furnace 2 has a length of 50 feet and the apparatus is arranged to process coils of bi-metal strip upto 500 feet (152m) long. The apparatus 1 is provided with a hardening heating unit 19, placed between the supply coil 15 and the right hand quenching zone 14 of the system. The hardening unit 19 comprises a furnace which heats the strip metal rapidly to an

5 austenitising temperature of 1180°C. The strip metal is quenched in the right hand quenching zone 14 so that it is cooled very rapidly so that martensite is formed in the metal structure and the metal is hardened. The subsequent passes through the furnace are done at a tempering temperature of

10590°C. The time any given section of strip material is held at the tempering temperature in the furnace is five minutes and is determined by the speed at which the bi-metal strip is passed through the furnace 2.

15 The graph shown in Fig. 2 shows a temperature/distance profile for the apparatus shown in Fig. 1. Examples of typical operational parameters for the system are given below in Table 1.

0

TABLE 1

Size Speed* Temp (°C) Soak Time

(mm) (ins/min) (m/min)

<27.5 x 0.9 80"/min (2m/min) 590° 5 mins

41.5 X 1.1 37"/min (0.94m/min) 575° 10.8 mins

67.5 X 1.6 18"/min (0.45m/min) 560° 22.2 mins

* Determined by speed of hardening furnace.

25

Total travel length approx 200ft/61m Time to pass through will be approx:-

<27.5mm = 80" per min 6.66 ft/min 2m/min 30 mins 41.5mm = 37" per min 3 ft/min 0.94m/min 66 mins 567.5mm = 18" per min 1.5 ft/min 0.45m/min 133.3 mins

Fig. 3 shows the five photo micrographs a - e (x 690 magnification) illustrating the optical microstructure produced in M42 bi-metal handsaw strip resulting from five 0 minute tempering at 590°C of metal strip hardened for 2 minutes at 1180°C and having a grain size (G/S) of 20/22 (Fig. 3a) . As will be seen from Fig. 3b-e the microstructure transforms gradually after each successive tempering stage resulting in a microstructure (Fig. 3e) broadly similar to 5 the microstructure of bandsaw strip (Fig. 3f) tempered by using known conventional process.

Fig. 4 is a graph showing the performance data for M42 bimetal bandsaw blade of cutting time against number of cuts 0 comparing a standard blade heat treated using conventional technique (STD H/T) and blades of the invention. It will be seen that blades having three and four tempering soaks of five minutes at 590°C have improved performance over blades tempered conventionally (indicated by STD H/T) . 5

Fig. 5 shows a life comparison (indicated by the number of cuts) of blades treated using different numbers of soak times and length of soak times compared against conventionally heat treated materials (STD H/Tl and 2) . 0

It will be appreciated that various different hardening speeds will be required by different material widths, thicknesses, types and various adjustments may be made to the timings and temperatures of the embodiment described herein above without departing from the scope of the present invention.

An equation known as the tempering parameter equation describes the relationship between the temperature and time for tempering of steel. P=T(20 + LOGt)

where P = the tempering parameter

T = the temperature in absolute degrees (°C plus 273) t = time in hours.

It has been found that a tempering parameter of P - 16347 is required to obtain a constant hardness of 67HRC. By substituting this value into the tempering parameter equation it is possible to calculate the precise temperatures required to accommodate the various tempering times required by materials having different thicknesses, widths, types etc. This information can also be used to calculate the temperatures that are required by the various different types of widths, thicknesses, types of bandsaw material.

Fig. 6 shows the relationship between tempering time (t) and temperature (T) to achieve a constant hardness of HRC67.

Fig. 7 is a graph of temperature against speed (ins/min and m/min) to achieve constant tempering parameter of 16347 (67HRC) for the in-line tempering of M42 bi-metal bandsaw material .

Claims

1. A tempering system (1) for use in the heat treatment of elongate strip metal (4) , which system comprises material heating means (2) formed and arranged for heating material to

5 be treated to a tempering temperature in a heating zone thereof; material quenching means (12,14) for cooling material heated to said tempering temperature; and material transport means (6) for conveying substantially continuously a substantially continuous elongate length of said material

10 to be heat treated sequentially through said heating zone (2) and said material quenching means (12,14), so that said material (4) is heated to said tempering temperature for a predetermined tempering period as it passes through said heating means (2) and is substantially cooled and hardened

15 upon passing through said quenching means (12,14) .

2. A tempering system according to claim 1 formed and arranged to provide at least two tempering cycles to an elongate strip metal (4) passing therethrough.

20

3. A tempering system according to claim 1 or claim 2 formed and arranged to provide a series of successive tempering heating zones (2) and quenching means (12,14) .

254. A tempering system according to any of claims 1 to 3 formed and arranged to recycle said strip (4) one or more times through a single tempering heating zone (2) and quenching means (12,14).

305. A tempering system according to any of claims 1 to 4 which is coupled in line with a hardening system (19) having a material hardening heating means for heating said material to a hardening temperature; said hardening heating means and said tempering system (1) being spaced apart by a quenching zone (14) .

56. A tempering system according to claim 5 wherein said hardening heating means (19) is formed and arranged to heat strip metal (4) passing therethrough to a substantially elevated temperature above the critical temperature of the metal so as to transform the metallurgy of the metal to 10 austenite.

7. A tempering system according to any of claims 1 to 6 wherein said tempering temperature is any temperature below the critical temperature where there is a change in the

15 metallurgical structure of the strip metal whereby retained austenite is transformed to martensite.

8. A tempering system according to any of claims 1 to 7 wherein said tempering temperature is in the range of from

20580 to 610°C.

9. A tempering system according to any one of claims 5 to 8 wherein said hardening temperature is in the range of from 1150 to 1190°C.

25

10. A tempering system according to any one of claims 5 to 9 wherein said material transport means (6) is formed and arranged with a material transport means of said hardening means (19) so that material passes through said hardening

30 means first and once only before entering the tempering system (1) .

11. A tempering system according to any one of claims 5 to 10 wherein said metal material (4) having been first heated in said hardening heating means (19) and quenched in said quenching zone (14) between said hardening heating means (19) and said material heating means (2) passes through said tempering heating zone at least twice.

12. A tempering system according to any of claims 1 to 11 wherein said tempering heating zone (2) of said material heating means comprises a single elongate furnace means (2) provided, at each end (8,10) thereof, with a quenching zone (12,14) of said quenching means and material transport return means (16) for recycling material exiting a said end of said tempering heating zone and quenched in said quenching zone, back into the tempering heating zone of said tempering heating means.

13. A tempering system according to any of claims 5 to 12 wherein said material transport means comprises at least one of material pull through apparatus; turntable; wind up system; coiling and uncoiling apparatus and a plurality of roller guides (6) formed and arranged for guiding metal material to be treated held in an untreated coiled form through said respective hardening, tempering and quenching zones and into a second treated coiled form.

1 . A tempering system according to any of claims 1 to 13 wherein said material transport means (6,16), said heating means (2) and quenching means (12) are formed and arranged so that two lengths of elongate strip metal (4) may be processed more or less continuously and simultaneously in side-by-side, WO 98/38346 _ , . _ PCT/GB98/00617 parallel relation.

15. A tempering system according to any of claims 1 to 14 wherein said tempering heating zone of said material means is

5 formed and arranged to heat material to be treated up to a temperature in the range of from 550 to 650°C.

16. A tempering system according to any of claims 1 to 15 wherein said material transportation means (6,16) is formed

10 and arranged for exposing said material to be treated to said tempering temperature in said tempering heating zone (2) of said material heating means for a period of time of from 4 to 30 minutes during the or each passage therethrough.

1517. A tempering system according to any of claims 1 to 16 wherein said quenching means (12,14) are formed and arranged for cooling rapidly material exiting said tempering zone to a generally ambient temperature, said quenching means being selected from the group including water, oil or blast chilled

20 air.

18. A method of tempering continuous extended lengths of strip metal (4) comprising the steps of subjecting said strip metal to at least one tempering cycle, each said tempering

25 cycle comprising feeding said strip metal through a tempering furnace (2) in a substantially extended form, for a short period of time at an elevated tempering temperature, sufficient for tempering said strip metal without softening thereof, and then through a quenching means (12) .

30

19. A method according to claim 18 wherein said strip metal is subjected to a plurality of tempering cycles.

20. A method of heat treating elongate strip metal, so as to increase its hardness which method comprises the steps of : providing a tempering system (1) according to any one of claims 1 to 17; passing said material (4) more or less continuously through said material heating means (2) and said quenching means (12,14) sequentially by said material transport means (6,16) so that said material is heated to a tempering temperature for a more or less predetermined period of time and cooled more or less rapidly.

21. A method as claimed in claim 20 which includes an additional first step prior to heating to said tempering temperature of heating to a substantially elevated hardening temperature and quenching.