Methods of Piercing and Enlarging Elongate Metal Members Such as Seamless Tubes

This invention relates to methods of piercing and enlarging metal at high temperature in contact with flowing metal surfaces.

The manufacture of seamless tube and pipe using piercer points, pipe plugs and reeler plugs is well known. Typically in a high mill a red hot tube is rolled over a stationary plug to reduce the wall thickness of the tube. The tube is pulled over the plug by means of two rolls grooved to the outside diameter of the tube. In a normal mill practice the wall is reduced in two passes with the tube being rotated 90.degree. between passes.

The hollow tube above mentioned is formed by feeding a heated billet over a piercer point between rolls which force the billet over the piercer point while forming the outer periphery of the pipe. Typically this operation is carried out on a Mannesmann piercing mill having two double conical rolls, each set at an angle to the work piece. A billet at about 1750.degree. F. to 2300.degree. F. is fed into the mill until engaged by the rolls which start it spinning. Since the rolls are set at an angle to the path of the billet, one component of force is in the longitudinal direction which pulls the billet forward onto the piercing point. The metal then flows along both sides of the piercer point forming a tube. After each billet is formed the piercer point is quenched in water. Thus the piercer point operates under extreme conditions of thermal shock, compression, wear, impact, longitudinal and torsional stress. Piercer points have in the past been generally made of an alloy of iron containing 0.2% carbon, 2% chromium and 2.5% nickel whereas high mill plugs have in the past been generally made of high carbon nickel chromium steels such as 1.75% carbon, 18% chromium, 6% nickel. Such plugs and points are cast, ground to size and scaled at 1750.degree. F. to 2000.degree. F. This scaling of the point or plug was essential to effective performance of the plug or point and provided a surface carburization protective scale without which the point or plug did not operate successfully. Such plugs and points had a relatively short work life at best and were rapidly rendered useless in the presence of slag inclusions in the steel. As a consequence it has been the practice to crop a large section from the head of each ingot to reduce the possibility of slag inclusions and the consequent reduction in the life of plugs and points. This is obviously economically undesirable.

We have discovered an apparatus and method of piercing metal using a piercer point, high mill plug, reeler plug or like member which does not need to be scaled before use, which has vastly increased service or work life and which is not adversely affected by the presence of slag in the metal being worked. Preferably such members are made of a composition relatively low in iron but high in cobalt. Compositions for use in such members lie in the broad range as follows:

In the foregoing composition we have found that cobalt is absolutely essential and that the carbon must be reduced as the iron content increases. We have also found that boron, molybdenum, tungsten, titanium, nickel, copper, vanadium, aluminum and tantalum and columbium may be present for imparting the qualities generally associated with such elements without detrimentally affecting the alloy. In the case of piercer points we prefer to use lower carbon contents, below 0.5%, whereas in the case of high mill plugs we prefer the higher range of the foregoing analysis, about 0.5% carbon.

Preferably, we maintain the analysis with a narrower range of composition for maximum hot hardness and resistance to erosion and deterioration as well as maximum surface qualtities such as resistance to slag or oxide accretion. Such narrower range of composition is:

Again various amounts of other alloy constituents as mentioned above may be present.

A preferred composition range including auxiliary alloys for piercing and/or enlarging a seamless tube is:

C about 0.015% to 2%

Co about 5% to 65%

Cr about 15% to 35%

Fe up to about 30%

Ni about 2% to about 20%

Mo about 1.5% to about 6%

Bo about 0.003% to about 0.01%

Ti up to about 0.25%

W about 15% to 18%

Si up to about 1.5%

However, the tungsten can be omitted while still producing a marked improvement.

The subject matter of this invention may perhaps best be understood by reference to the folowing examples.

Three 51/8 inches high mill plugs were cast from the composition: ##EQU1## These plugs were used four times and reground for a fifth use. The recapitulation of usage is as follows:

Total pieces rolled to date on three plugs is 2765 for an average of 920 pieces per plug and the plugs are still usable. The normal number of pieces on the same mill using a conventional plug of high carbon nickel chromium steel is 66 to 70 pieces per plug. It is apparent that the plugs of this invention are about 15 times better than conventional plugs in pieces per plug produced. The significance of this in time and expense is obvious.

Four high mill plugs 4 11/16 inches in size were cast from the following composition: ##EQU2## These plugs were tested in making 41/2 inches O.D. pipe, wall thickness 0.25, inch weight per foot 11.6 No. of alloy steel range 2 in the usual manner. The pieces rolled per plug in one run are listed in the following table:

The total pieces rolled in one run was 913 or an average of 228.2 pieces per plug. The plugs are still in good condition and were reground for further use. Here again, the normal production for conventional plugs is about 65 pieces.

Six piercer points were cast from a composition of the following analysis: ##EQU3## These points were solution annealed and aged at 1400.degree. F. for 8 hours. These points average 366 billets on poor quality (slag inclusions) steel whereas standard points lasted an average of 1.4 billets. This illustrates the ability of the steel of this invention to stand up in the presence of slag which heretofore has been destructive of piercer points.

Eight piercer points 33/4 inches diameter were cast of material having the following analysis: ##EQU4## and air cooled. These points were used in a Mannesmann piercing mill using an air quench to pierce billets of a coarse grain open top casting heat of steel of the following analysis: ##EQU5## These points pierced a toatal of 2200 billets for an average of 275 passes per point. Standard points on the same material ran 455 points for 3699 billets or 8.1 passes per point.

Four points of the analysis of Example IV were used in the same mill but water quenched. The four points produced 875 billets pierced for anverage of 219 passes per point as compared with the 8.1 passes of the conventional points.

Eight piercer points were cast of an alloy of the composition: ##EQU6##

The points were subject only to normal shake out treatment and used in same mill and on same material as Example IV. The eight points pierced a total of 1957 billets, using an air quench, for an average of 245 passes per point. This compares with 8.1 passes per point of conventional points as pointed out in Example IV.

Seven piercer points were cast of a material of the composition: ##EQU7## The points were solution annealed and placed in same mill as Example IV piercing the same material but using a water quench. The seven points pierced 2134 billets for an average of 305 passes per point as compared with 8.1 passes per point of conventional materials.

Eight piercer points of the following analysis were cast: ##EQU8## These points were used in the as cast condition to pierce billets on the same mill as Example IV to pierce billets from a different heat from Example IV but cast to the same specification. The points were air cooled and produced 2200 billets for an average of 275 passes per point. As a comparison 41 standard points were used to pierce 75 billets of the same heat for an average of 1.8 passes per point.

Four points were cast from an alloy having the analysis: ##EQU9## These points were solution annealed and aged at 1450.degree. F. for 8 hours and used in same mill as Example IV to pierce billets from a like heat of steel. The points were used in rotation with wter quenching for a total of 1486 passes for an average of 366 passes per point. As was pointed out in Example IV conventional points were useful for an average on only 8.1 passes.

Seven points were cast from an alloy whose composition was: ##EQU10## These points were solution annealed and used in same mill as Example IV on a like heat of steel. The seven points pierced a total of 2490 pieces for an average of 356 passes per point. The points were still usable at the end of the run. Standard point life was less than 2 passes per point.

Four piercer plugs were prepared (27/8 inches) from the following composition: ##EQU11## These plugs were run alongside standard piercer plugs. The plugs of this invention produced an averge of 74 tubes per plug and were still usable. The standard plugs were scrapped as not usable at an average of 40 tubes per plug.

From the foregoing examples it will be evident that the method of piercing according to our invention will produce many more pipes than conventional methods using conventional pipe plugs and piercer points and in some cases, as where the metal contains slag, the method of our invention will consistently produce products where the conventional points are unable to be used.

We believe that the peculiar properties of our piercer method and metal working members comes from the formation of cobalt oxide or cobalt-chromium oxide film under high temperature and pressure which acts as a lubricant. Peculiarly no noticeable film of oxide is formed on heating to high temperatures, but a dark film forms on working at the same temperatures, and only on that portion of the tool in contact with the work. The working temperatures of tools according to our invention is about above 1400.degree. at the working surface as compared with about 500.degree. F. for conventional tools. At 1400.degree. F. tools of our invention have a hardness of about 170 DPH while conventional tools are only about 45 DPH. The foregoing theory appears to be supported by the vastly improved and unique results of our invention, but it is at most a theory and we do not wish to be bound by it but rely on the unique result achieved regardless of theory.

While we have illustrated certain preferred embodiments and practices of our invention it will be understood that this invention may be otherwise practiced within the scope of the following claims.