Various processes have been known for the production of wire mesh screens. The use of strong high carbon steel mesh screens is particularly desirable for use in mining, quarry, sand and gravel, steel milling, coal coking and slag operations. Also, the use of such mesh screens is important for security barriers in high security areas. High carbon steel screens or mesh are generally made by processes involving weaving pre-crimped wires into a mesh by the use of a loom or the like. The weaving machines and techniques used are substantially the same as in textile machine usage. This method, however, is not particularly suitable in making steel mesh screens using wires with a substantially large diameter. The use of weaving methods therefore has been utilized generally in the manufacture of steel screens with wires of relatively small diameters. When it was necessary to construct wire mesh screens from wires having large diameters (over about 0.03 inches) usually this was accomplished manually.
In earlier filed U.S. Pat. No. 4,686,342 (Collier et al) which is commonly owned with the present invention, a method for making steel wire mesh screens from high carbon steel wires was disclosed and claimed. Collier et al disclosed a process whereby an oil quench was required at the conclusion of the process. In addition, Collier et at required the formation only of martensite in the welded mesh by an elevated heating post weld step. During continued development of this process which resulted in the present invention, it was found that not only was the time consuming oil quenching not required, but also the formation of martensite by itself was not optimum.
There are several processes relating to steel wire structures such as those disclosed in U.S. Pat. Nos. 3,539,752; 3,884,730 and 3,648,006.
The process of U.S. Pat. No. 3,648,006 differs substantially from the present invention in that a single wire is heated by direct current heating between two electrodes separate and distinct from the welding electrodes. Even separate transformers are used, Th for heating and Tw for welding. The principle of the process is similar to Ernst (U.S. Pat. No. 3,539,752) in that significant pre-heating will leave some residual heat after welding which will slow the rate of cooling. However, U.S. Pat. No. 3,648,006 only heats one of the wires to be welded. It is assumed that the other wire to be of similar material and therefore would not derive much heat from a point contact with a pre-heated wire. It is foreseen that by pre-heating to 1,000.degree. C. the wire to be bent and wrapped would be softened and thus the subsequent bending would require less force. This prior art patent also relates to welding circular wire cages for reinforcing unlike the present application which relates to welding flat sheets of wire mesh. This prior art patent describes a process for welding one wire, whereas the present invention covers a process for welding several wires simultaneously.
U.S. Pat. No. 3,884,730 relates to surface hardening by induction or flame hardening of machine elements, such as injection molding lead screws, which is quite unrelated to welded wire mesh screens. The U.S. Pat. No. 3,884,730 also relates to treatment of expensive tool steels, having chromium, vanadium, molybdenum and tungsten in significant amounts whereas these elements are not present, other than as trace elements, in the grades of steel covered by the present application.
The process described in the Ernst U.S. Pat. No. 3,539,752 does not encourage the presence of martensite in the final weld nugget, but rather is designed to eliminate martensite. The Ernst patent does not refer to high carbon steel either directly or by interference. The Ernst patent does not refer to martensite or pearlite or any other austenite products. The Ernst patent does not refer to the size of the bars being welded. The Ernst process differs in method of applying pre-heat and post-heat to the welded junction. Ernst uses hot gasses while applicants use controlled electric current passage. The Ernst patent heats slowly while applicants heat fast (15 cycles is typical for our process) and in a "gradual" cooling is the object whereas in the present invention a defined, controlled rate is used which is dictated by the appropriate transformation curve for the steel being welded. The Ernst process is not practical in that the hot air or gasses tend to heat adjacent machinery so that these items have to be either shielded or cooled. Applicants are not familiar with any mesh welding machine equipped with the process described by Ernst, i.e., after 25 years it is still not practical or acceptable or does not seem to have advantages.
A major reason why none of the above-discussed prior art deals with welding of high carbon steel is that in prior art attempts the wire has become weak and brittle in the vicinity of the weld. Thus, the processes involved with welding of wire have been limited to wires containing low carbon steel.
Welding operations usually necessitate the local application of heat, the amount of heat applied and the temperature locally attained depend upon the type of weld to be made. Thus, the temperature at the heated area may be only sufficient to render the meeting surfaces of the members semi-plastic as in forge welding or sufficient to melt the meeting surfaces thoroughly as in full fusion welding. Although in welding operations the heat is usually applied only to or adjacent to the surfaces to be united, metal adjacent to and remote from the heated surfaces is also heated by conduction from the directly heated surfaces. Such heating being uneven tends to warp and distort the welded article or, in some cases, causes brittleness.
The problem of shrinking and warping is particularly important in the fabrication of welded structures from large sheets and plates such as those which are now used in the construction of railway box cars, gondola cars and other rolling stock. As the metal sheets and plates used in this particular field are relatively thin, they are peculiarly susceptible to warping during welding as the small mass of metal is not strong enough to resist stresses developed by the expansion and contraction incidental to the localized application of heat to the metal plate or metal object.
As the grain size of the steel is influenced by the time interval during which the temperature is maintained in or above the upper critical range, it is desirable to cool the metal rapidly to below the upper critical range as soon as possible after it has been welded. In cross wire welding similar to projection welding, a large current is passed through the two surfaces which are to be welded. As the time during which this current passes is brief, generally less than a second, this gives rise to severe temperature gradients in the heat affected zone. The effects of these temperature gradients on steel having a carbon content of 0.5% and greater, are deleterious.
In U.S. Pat. No. 4,686,342 a process is claimed whereby high carbon steel wire mesh is welded, heated after welding, by induction or a similar process and subsequently quenched by oil or an equivalent quenching medium. This was found in practice to be unfeasible due to the mesh being fed through an oil quench curtain in an intermittent manner, in increments of a distance equal to the opening of the said mesh plus a wire diameter. In this situation the mesh would be stationary for a period of several seconds, typically five seconds. This form of index feeding hot mesh through a quenching curtain causes a severe temperature gradient to occur and varying cooling rates and thus it was deemed to be impractical. Therefore, it was decided by the applicants to utilize the residual heat in the heat-affected zone and control the cooling rate to achieve a majority of pearlite, 50% minimum (based on total volume) in the finished weld and adjacent steel. This dominant amount of pearlite would result in an "annealed" heat-affected zone. The mechanics of pearlite and advantages of its presence are fully discussed herein.
The manganese content of the steel being welded is significant. Increased manganese content moves the transformation diagram which increases the time required for successful welding. It has been found that steels with a low manganese content, typically less than 0.8%, generally are easier to weld.
The relationship of temperature and time on the transformation of steel depends on the chemical constituent of the steel and the transformation curves or S curves are readily available for known alloys.
To achieve small grain size, that is, one with many boundaries to facilitate the formation of pearlite which forms along the grain boundaries, it is necessary to understand the changing relationships which occur throughout the weld cycle.
It is conceivable that given the right circumstances of market and some technology being developed, very high speed welding of high carbon wires into light, that is wires under 0.032" in diameter, could be an attractive reality.
This particular segment of the industry is currently served by weaving wires into mesh, having various weaves, simple, twill, arched, etc. It is possible with light wires to weave using automatic methods, similar in style to the textile industry; one of the foremost of these is the Emil Jaeger Co., Germany.
With respect to welding, the general approach foreseen would be to use a seam or roller type of welding system. Roller welding machines currently in use in the fabrication of low carbon wire assemblies can achieve speeds up to 600 feet per minute; typical speeds for automatic weaving machines would be 1-24 inches/minute.
In order to adapt roller welding technology to high carbon steel wire mesh, it is foreseen that welding speeds would be reduced to satisfy the metallurgy, that is the time for transformations to occur. It should, however, be possible to achieve speeds of 100 feet per minute. One of the mechanical problems to be solved would be to feed sufficient cross wire. For example, a 40 mesh would require 40 wires per inch.times.12 per foot.times.100 feet per minute=48000 wires/minute. Each wire would need to be straight and positioned correctly for welding.
There is considerable global market for fine wire mesh and to be able to produce 50 to 100 times faster than at present could be justification to proceed with the design of the present invention.