The Working of Steel / Annealing, Heat Treating and Hardening of Carbon and Alloy Steel

by Fred H. Colvin; K. A. Juthe

PICKLING THE FORGINGS

The forgings were then pickled in a hot solution of either niter cake or sulphuric acid and water at a temperature of 170°F., and using a solution of about 25 per cent. The solution was maintained at a constant point by taking hydrometer readings two or three times a day, maintaining a reading of about 1.175. Sixty forked or one hundred single rods were placed in wooden racks and immersed in a lead-lined vat 30 by 30 by 5 ft. long. The rack was lowered or lifted by means of an air hoist and the rods were allowed to stay in solution from 1/2 to 1 hr., depending on the amount of scale. The rods were then swung and lowered in the rack into running hot water until all trace of the acid was removed.

The rod was finally subjected to Brinell test. This shows whether or not the rod has been heat-treated to the proper hardness. If the rods did not read between 241 and 277, they were re-treated until the proper hardness is obtained.

CHAPTER IV

APPLICATION OF LIBERTY ENGINE MATERIALS TO THE AUTOMOTIVE INDUSTRY[1]

[Footnote 1: Paper presented at the summer meeting of the S. A. E. at Ottawa Beach in June, 1919.]

The success of the Liberty engine program was an engineering achievement in which the science of metallurgy played an important part. The reasons for the use of certain materials and certain treatments for each part are given with recommendations for their application to the problems of automotive industry.

The most important items to be taken into consideration in the selection of material for parts of this type are uniformity and machineability. It has been demonstrated many times that the ordinary grades of bessemer screw stock are unsatisfactory for aviation purposes, due to the presence of excessive amounts of unevenly distributed phosphorus and sulphide segregations. For this reason, material finished by the basic open hearth process was selected, in accordance with the following specifications: Carbon, 0.150 to 0.250 per cent; manganese, 0.500 to 0.800 per cent; phosphorus, 0.045 maximum per cent; sulphur, 0.060 to 0.090 per cent.

This material in the cold-drawn condition will show: Elastic limit, 50,000 lb. per square inch, elongation in 2 in., 10 per cent, reduction of area, 35 per cent.

This material gave as uniform physical properties as S. A. E. No. 1020 steel and at the same time was sufficiently free cutting to produce a smooth thread and enable the screw-machine manufacturers to produce, to the same thread limits, approximately 75 per cent as many parts as from bessemer screw stock.

There are but seven carbon-steel carbonized parts on the Liberty engine. The most important are the camshaft, the camshaft rocker lever roller and the tappet. The material used for parts of this type was S. A. E. No. 1,020 steel, which is of the following chemical analysis: Carbon 0.150 to 0.250 per cent; manganese, 0.300 to 0.600 per cent; phosphorus, 0.045 maximum per cent; sulphur, 0.050 maximum per cent.

The heat treatment consisted in carbonizing at a temperature of from 1,650 to 1,700°F. for a sufficient length of time to secure the proper depth of case, cool slowly or quench; then reheat to a temperature of 1,380 to 1,430°F. to refine the grain of the case, and quench in water. The only thing that should limit the rate of cooling from the carbonizing heat is distortion. Camshaft rocker lever rollers and tappets, as well as gear pins, were quenched directly from the carbonizing heat in water and then case-refined and rehardened by quenching in water from a temperature of from 1,380 to 1,430°F.

The advantage of direct quenching from the carbonizing heat is doubtless one of economy, and in many cases will save the cost of a reheating. Specifications for case hardening, issued by the Society of Automotive Engineers, have lately been revised; whereas they formerly called for a slow cooling, they now permit a quenching from the pot. Doubtless this is a step in advance. Warpage caused by quenching can be reduced to a minimum by thoroughly annealing the stock before any machine work is done on it.

Another advantage obtained from rapid cooling from the carbonizing heat is the retaining of the majority of the excess cementite in solution which produces a less brittle case and by so doing reduces the liability of grinding checks and chipping of the case in actual service.

In the case of the camshaft, it is not possible to quench directly from the carbonizing heat because of distortion and therefore excessive breakage during straightening operations. All Liberty camshafts were cooled slowly from carbonizing heat and hardened by a single reheating to a temperature of from 1,380 to 1,430°F. and quenching in water.

Considerable trouble has always been experienced in obtaining uniform hardness on finished camshafts. This is caused by insufficient water circulation in the quenching tank, which allows the formation of steam pockets to take place, or by decarbonization of the case during heating by the use of an overoxidizing flame. Another cause, which is very often overlooked, is due to the case being ground off one side of cam more than the other and is caused by the roughing master cam being slightly different from the finishing master cam. Great care should be taken to see that this condition does not occur, especially when the depth of case is between 1/32 and 3/64 in.

CARBON-STEEL FORGINGS

Low-stressed, carbon-steel forgings include such parts as carbureter control levers, etc. The important criterion for parts of this type is ease of fabrication and freedom from over-heated and burned forgings. The material used for such parts was S. A. E. No. 1,030 steel, which is of the following chemical composition: Carbon, 0.250 to 0.350 per cent; manganese, 0.500 to 0.800 per cent; phosphorus, 0.045 maximum per cent; sulphur, 0.050 maximum per cent.

To obtain good machineability, all forgings produced from this steel were heated to a temperature of from 1,575 to 1,625°F. to refine the grain of the steel thoroughly and quenched in water and then tempered to obtain proper machineability by heating to a temperature of from 1,000 to 1,100°F. and cooled slowly or quenched.

Forgings subjected to this heat treatment are free from hard spots and will show a Brinell hardness of 177 to 217, which is proper for all ordinary machining operations. Great care should be taken not to use steel for parts of this type containing less than 0.25 per cent carbon, because the lower the carbon the greater the liability of hard spots, and the more difficult it becomes to eliminate them. The only satisfactory method so far in commercial use for the elimination of hard spots is to give forgings a very severe quench from a high temperature followed by a proper tempering heat to secure good machine ability as outlined above.

The important carbon-steel forgings consisted of the cylinders, the propeller-hubs, the propeller-hub flange, etc. The material used for parts of this type was S. A. E. No. 1,045 steel, which is of the following chemical composition: Carbon, 0.400 to 0.500 per cent; manganese, 0.500 to 0.800 per cent; phosphorus, 0.045 maximum per cent; sulphur, 0.050 maximum per cent.

All forgings made from this material must show, after heat treatment, the following minimum physical properties: Elastic limit, 70,000; lb. per square inch, elongation in 2 in., 18 per cent, reduction of area, 45; per cent, Brinell hardness, 217 to 255.

To obtain these physical properties, the forgings were quenched in water from a temperature of 1,500 to 1,550°F., followed by tempering to meet proper Brinell requirements by heating to a temperature of 1,150 to 1,200°F. and cooled slowly or quenched. No trouble of any kind was ever experienced with parts of this type.

The principal carbon-steel pressed parts used on the Liberty engine were the water jackets and the exhaust manifolds. The material used for parts of this type was S. A. E. No. 1,010 steel, which is of the following chemical composition: Carbon, 0.05 to 0.15 per cent; manganese, 0.30 to 0.60 per cent; phosphorus, 0.045 maximum per cent; sulphur, 0.045 maximum per cent.

No trouble was experienced in the production of any parts from this material with the exception of the water jacket. Due to the particular design of the Liberty cylinder assembly, many failures occurred in the early days, due to the top of the jacket cracking with a brittle fracture. It was found that these failures were caused primarily from the use of jackets which showed small scratches or die marks at this joint and secondarily by improper annealing of the jackets themselves between the different forming operations. By a careful inspection for die marks and by giving the jackets 1,400°F. annealing before the last forming operation, it was possible to completely eliminate the trouble encountered.