IE841436L - Improving the bendability of copper pipe - Google Patents

Abstract

1. A process for improving the bendability of a cold-worked copper pipe characterised in that the cold-worked copper pipe is subjected to a partial annealing step, which affects the cortical zone only, at a temperature of between 150 and 350 degrees C for a period of between 5 minutes and 24 hours.

Description

ay a a » The present invention concerns a method for improving the bendability of hardened copper piping or tubing. The invention applies more particularly to straight cold-worked copper pipes, used in sanitary installations whose external diameter is generally between 8 and 22 5 millimetres, and whose wall thickness is generally 1 millimetre.

In practice these pipes call for processes of bending which are effective whether the work 1s done in a builder's yard with manual bending machines or in a factory workshop with automatic bending 10 machines operated hydraulically or pneumatically.

Straight copper pipes are generally manufactured from a tubular blank of large diameter obtained by hot drawing and piercing, then cold rolling and drawing this blank until the final dimensions are reached, 15 without any intermediate annealing.

The work hardening of the finished pipes is thus very important and the mechanical properties under tension are on averager- Breaking strain Rm Elastic limit R0.2 Elongation at breaking (% with 200 mm sample) Rm = 450 MPa = 430 MPa = 2% to 4% in the case of copper of the said quality Cu/b, having 99.90% Cu, reduced with phosphorus.

Because of this low ductilty, bending, especially if done with pneumatic machinery whose action is very rough, tends to give rise to cracking, incipient weaknesses and even some fractures.

Further it must be borne in mind, that the use of tubing softened by annealing is not acceptable because of its excessive ductility leading to inelegant deformation in the case of fittings where the pipe is visible.

It would therefore be of interest to find a method which would permit some increase in the ductility of straight pipes, and thereby to obtain normal bending without any faults, even with bends having a small radius of curvature, and using bending machines with a rapid action, but stopping short, however, of complete annealing for the reason stated above.

This need has led to the discovery that this result can be obtained by a partial heat annealing process, which, surprisingly, only affects the external cortical zone of the surface of the pipe, and leaves interior zones unchanged. The annealing is carried out at low temperatures of between 150° and 350°C and for a total treatment time between 5 minutes and 24 hours. Preferably treatment is maintained for from 15 minutes to one hour, the temperature being meanwhile maintained between 260° and 310°C.

Figure 1 illustrates the micrographic appearance of a pipe treated according to this invention.

A scries of tests have been carried out. on some standard pipes measuring 14 mm in external diameter and 1 mm wall thickness, formed by drawing, consequently work hardened, having the following mechanical properties:- Breaking strain Rm = 450 MPa Elastic limit Rg,? = 430 MPa Elongation at break A^og = 2.5% Heat annealing was carried out in a hot air oven with air circulation under the following conditions: mins at 270° - 300° - 310°C 1 hour at 260° - 280° - 300°C 16 hours at 210° - 240° - 260°C Following these annealing processes, it was found that:-the breaking strain Rm lay between 390 and 420 Mpa the elastic limit Ro.2 1dy between 370 and 400 MPa the elongation at break A20O between 4% and 6%.

Exceeding the stated time and temperature ranges, rapidly produces general softening, giving the typical mechanical characteristics of the fully annealed state with: Breaking strain Rm = 250 MPa Elastic time Rq.2 = 60 MPa Elongation at break - Agoo = ^8% to 42% which is not acceptable for sanitary fittings exposed to view.

If one examines by micrograph (in longitudinal section) a pipe (1) roughly cold-hardened and a pipe partially annealed according to the invention, it can be observed that: the untreated cold-hardened pipe as manufactured, shows longitudinal fibres consisting of bands of fibrous material lying at <111 angle of 3b" to the axis of the pipe. the pipe partially annealed according to the invention (Fig. 1), is unchanged through Its entire thickness, except for the external zone of the pipe surface (3) which is recrystallised to a fine texture to a depth of the order of 0.02 mm, which is a completely unforseen result.

For longer periods and/or higher temperatures, the recrystallisation progresses gradually deeper changing the structure of the whole thickness of the pipe.

Some bending tests have been carried out on pipes treated according to the invention, on a manual bending machine equipped with bending rollers allowing bends of radius (r = 38 mm) well below those obtained with the normal rollers on this equipment (r « 50 mm). on a pneumatic workshop bending machine used in the most severe conditions (rollers supplied with the machine to make bends of small radius).

In all cases a significant improvement of the bending characteristics (reduction of cracks and incipient cracks) has been observed with the treated pipes in comparison with untreated pipes as manufactured.

The recrystallisation of the external surface zone inhibits the development of a constriction on the external fibres of the pipe during bending, cuts out practically all superficial flaws, whereas the elastic limit remaining high, the pipes retain good rigidity which allows them to be used under the same conditions as the hardened straight pipes used up to the present time.

Claims (4)

1. A method for Improving the bendability of a told-worked copper pipe, wherein the cold-worked copper pipe is subjected to a partial annealing step affecting only the external cortical zone of the surface of the pipe at a temperature of between 150 and 350°C for a period of between 5 minutes and 24 hours.

2. A method according to Claim 1, wherein the annealing step 1s carried out at a temperature of between 260 and 310°C during a period of between 15 minutes and 1 hour.

3. A method of treating a copper pipe substantially as hereinbefore described by way of Example.

4. A copper pipe whenever treated by a method as claimed in any of claims 1 to 3. Dated this the 8th day of June, 1984. 5, Dartmouth Road DUBLIN 6.