Elements Of Surface Hardening (1945)

many steel parts require a very high surface hardness and at the same time must have a tough core obtaining surface hardness like quench hardening a steel of high carbon content would sacrifice toughness this difficulty can be overcome by taking low carbon steel and increasing the carbon content of the surface region then after heating and quench hardening the skin of the steel attains a much higher hardness than the interior section with its lower carbon content here for example is a stop cylinder which require surface hardness and internal toughness this is the ink treatment specified carburizing hardening and tempering to the desired final hardness a surface hardness of Rockwell C 60 to 63 the test reveals that the present hardness is 20 Rockwell C this is lower than the hardness desired at either the surface or the core to carburized them the parts are placed in a container they are surrounded by a carburizing medium such as energized charcoal a material composed largely of carbon the parts in the container are completely covered with the charcoal tapping the container settles the charcoal more evenly around the part the container must be sealed airtight to prevent oxidation of the parts during the carburizing treatment to do this a cover is placed on top of the charcoal and carefully cement it in place with fire clay the sealed container is placed in the carburizing furnace where it will be held at red heat for a considerable length of time carburizing requires from 8 to 16 hours at a temperature of about 1700 degrees Fahrenheit here we see what happens under the influence of the elevated and sustained temperature the carbon in the energized charcoal migrates to and penetrates the surface of the part it encloses forming a skin of high carbon steel called the Kate the carbon penetrates to a depth of about 1/16 of an inch to carburized to this depth on this part takes about 12 hours after the container has been removed from the furnace and cooled the carburized parts are taken out after cleaning they are returned to the furnace for hardening to achieve full hardening the parts are needed to above the upper critical temperature then they are taken out and quenched in oil with some types of steel water is the quenching medium the parts are purposely hardened somewhat above the specified hardness subsequently tempering will achieve the desired toughness and hardness if the requirements and sizes of the parts to be kinked hardened warrant the gas carburizing method may be used it is just as efficient as pack carburizing convenience in mechanical handling determines the method used large pieces lend themselves better to pack carburizing while small parts like these bearing rollers are handled more readily in gas here the small parts are charged from a hopper into a rotary type furnace in which they will be tumbled at high temperature in an atmosphere of carburizing guests ordinary City gas is suitable for this purpose the gas will impart an additional carbon content to the surfaces of the part in essentially the same manner as the charcoal does in the tack method the furnace opening is sealed airtight and the exhaust gases are ignited this method requires a somewhat longer time than pack carburizing after the required number of hours the furnace is reopened one of the advantages of gas carburizing over the plank method is that the parts do not have to be cleaned but can be quench hardened directly from this furnace the parts are suited into a quench tank of oil sunk in the floor a few test pieces being removed and quenched separately there are numerous industrial applications where an extremely thin surface of fie hardness is adequate these levers for example require a hard case but only to adapt to a few thousands of an inch such a case can be obtained quickly in a bath of molten cyanide at a temperature between 1400 and 1500 and 50 degrees Fahrenheit these parts obtain their thin case during a brief immersion in the hot cyanide solution the cyanide as a carburizing action on the steel not unlike the action in solid or gaseous carburizing after 10 to 15 minutes the parts are removed from the cyanide that they are quenched in water some kinds of steel would be quenched in oil water has the advantage of washing off the cyanide also it produces a harder but more brittle surface than oil the quench completes the case hardening nitriding is a highly effective method of kinks hardening finished parts however it can be used only with certain alloy Steel's notably those containing aluminum chromium or molybdenum in this process steel parts are heated in a furnace in a stream of ammonia gas fed to a closed retort containing the parts the buffle acts as a flow indicator nitriding requires from one to three days in the furnace it takes place at a temperature between 900 and 1000 degrees Fahrenheit followed by partial cooling before the parts are removed in this case the parts which were put in the furnace Tuesday at 5:00 p.m. are taken out on Friday at the same time under heat the ammonia gas circulating over the part that decomposed forming nitrogen and hydrogen the nitrogen has combined with some of the elements of the steel to form a very hard nitrated case since the nitriding temperatures are low distortion of these precision parts is negligible nitriding can achieve hardnesses exceeding those obtainable by any other known treatment of Steel through this treatment parts attain a hardness range of 72 to 80 on the rockwell tester the highest hardness obtained by other method is about 68 flame hardening unlike the surface hardening processes previously observed does not add elements to the steel heating is achieved with an oxy acetylene torch and is followed immediately by water cooling this method is especially suited to large parts where only selected areas need to be hardened the torch flame moves slowly along the surface to be hardened heating it to the quenching temperature and is followed immediately by a spray of water which quenches the heated zone here is a cross-section diagram of a flame hardened gear-tooth it has been hardened only on specified areas on either side of the tooth surface the treated zones are the areas where the gear is most subject to wear somewhat similar principles are involved in an electrical method in which the surface heating is accomplished by high frequency induction a current of high frequency ranging from two thousand to one hundred thousand cycles per second flows through this coil inducing currents in the part which heat the surface layer to be hardened extremely rapid the heating is completed in a matter of seconds the current shuts off automatically and the part is immediately quenched with a water spray to review carburizing Zion IDing and nitriding add elements to the steel to achieve a surface hardness higher than that obtainable with the original steel that carburizing utilizes energized charcoal under the influence of high sustained heat to form a skin of high carbon content on the part gas carburizing also produces a high carbon case it is a process better suited to small parts syan iding is a method of obtaining an extremely thin hard case by immersing steel in a molten brass nitriding possible only with certain alloy steels is a process in which steel is heated in a furnace to which ammonia gas is being fed it can achieve a case hardness higher than obtainable by any other method flame hardening and induction hardening do not add elements to the steel and can produce only that hardness of which the original steel is capable flame hardening with the oxy acetylene torch is an effective method of hardening selected areas of apart induction hardening is a very rapid means of kinks hardening where the heat is induced by an electric current of high frequency by these methods steel parts can be given a hard case either on their entire surface or on a selected area only and yet retain their essential toughness