Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/06—Surface hardening
  • C21D1/09—Surface hardening by direct application of electrical or wave energy; by particle radiation

Definitions

• This invention relates to a method of transformation hardening of metals.
• Some metals can be hardened by heating them to a temperature in which a phase change occurs in the crystalline structure of the metal and then quickly cooling the metal to a temperature the phase transformation temperature so as to modify the crystalline structure of the metal.
• Important metals which can be hardened in this way include steel, alloys of titanium containing molybdenum, aluminium bronzes, copperaluminium alloys, and copper-silver-gold alloys. In the remainder of the specification the heat treatment technique will be described with reference to steel but it is understood that it is equally applicable to other metals susceptible of transformation hardening.
• the simplest known method for hardening steel is to heat it to a temperature of about 800o C and then quickly reducing the temperature by quenching in water or oil.
• This technique produces a relatively hard layer on the surface of the metal since the rate of cooling is much faster at the surface. It has the disadvantage however that serious distortion of the shape of material can occur and if tolerances are high it is necessary to further machine the work piece after quenching.
• Another technique which is commonly used is to heat the work piece with a high frequency induction heater but whilst the surface of the work piece is initially heated, the rate of energy input is comparatively slow so that heat is conducted inwardly before quenching and thus the work piece is prone to distortion, as before.
• a method of transformation hardening the surface of a metallic work piece comprising the steps of establishing an electric arc between a non-consumable electrode and a surface of the work piece, maintaining the arc on a region of the surface at an intensity and for a time sufficient to cause a phase transition in the region but not to melt the surface, and removing the arc from said region whereby the surface is cooled to below said transformation temperature and tranformation hardening occurs.
• the method includes the step of causing relative movement between the surface and the arc thereby governing the duration for which the arc is applied to a particular region on the surface.
• one or more arcs may be used to scan over a surface to harden the entire or a substantial part of the total surface of the work piece.
• one region is subjected to an arc it is preferred that the adjacent region is subjected to the influence of an arc before the first region has cooled to below its transformation temperature. Otherwise, the heating of the second region will cause annealing of the hardened material in the adjacent region.
• the pitch of the locus of the centre line of the arc is chosen so that the edges of the convolutions co-incide or slightly overlap. In this way, the hardness occurs across the entire surface. It has been found that by decreasing the pitch value so that there is significant overlap between adjacent convolutions the depth of penetration of the hardened zone can also be made substantially uniform. The harden is again constant but the depth of penetration is uniform. It is important to arrange for the adjacent convolutions of the trace to be made before there is substantial cooling of the previously made convolution. In this way, the arc does not anneal those regions which have just been subjected to the arc.
• the nature of the arc affected zone depends upon the current, relative speed of traverse of the arc, the gap between the electrode and the work surface, and the characteristics of the material being treated.
• the gap between the electrode and the work piece has been varied experimentally from between 0.1 cm and 2.4 cm and the speed of traverse has been varied from 0.25 cm/sec to 160 cm/sec.
• the cross-section of the arc at the work piece surface can be contracted or expanded.
• the width of the heat treated track could be varied from about 2 to 20 mm.
• the speed of traverse of the arc the depth of hardened zone could be varied from 0.01 to 0.2 cm.
• a test piece of 0.5% plain carbon steel was traversed by a 200 amp arc with a gap of 0.22 cm from the work piece.
• the speed of traverse being 41.3 cm/sec. This speed of traverse is roughly one hundred times faster than normal welding speeds.
• the current was chosen so that there was virtually no superficial melting at the surface of the work piece.
• the material was subsequently examined and it was found that the structure of the steel in the track was martensitic and there is a sharp interface between the martensitic region and the remainder of the work piece which is mainly of pearlite structure. Small volume of ferrite in the martensitic area were found to remain continuouswith ferrite of the unaffected pearlite area. With increasing proximity to the free surface the ferrite areas tend to disappear and be substituted by lighter etching martensitic regions so that the originally ferritic areas are still identifiable because, probably, these areas are still carbon deficient.
• a plain 0.2% carbon steel test piece was subjected to a current of 130 amps and a traverse speed of 12.6 cm/sec. With the slower speed of traverse the tranformed zone became thicker. The pearlitic areas within the zone became martensitic, as before. Closer to the free surface of the test piece the proportion of the section occupied by martensite increased due to carbon diffusion but nevertheless the significant proportion of the cross section is ferritic at the free surface.
• grey cast iron was subjected to a current of about 200 amps and a traverse speed of about 40 cm/sec. It was found on subsequent examination of the sample that the structure of the metal near the inner boundary of the transformed zone was uniformly martensitic except for graphite flakes. Near the free surface there were areas of high carbon martensite with retain austenite associated with graphite flakes which show indications of significant solution of the graphite during the passage of the arc.
• reducing the arc current to 140 amps and the traverse speed to about 0.3 cm/sec and increasing the distance between the electrode and the work piece to 0.95 cms a wider and thicker hardened region can be produced which tends to have a more even microstructure.
• the treated zone was found to consist of nodules of graphite, martensite and small areas of ferrite.
• the material is rather similar to the structure of a conventionally hardened nodular iron.
• Another factor which can influence the microstructure of the hardened zone is periodic fluctuations in the power supply to the arc.
• the successive current peaks of a single phase full wave rectified 50 Hz supply (having a repetition rate of 100 Hz) supplying a TIG torch interacts with the surface at 0.5 cm intervals.
• the table set out below summarizes the results of other trials which have been carried out.
• the work piece comprised a steel bar of 487 mm diameter, the steel being s.a.e. code 1040.
• the electrode tip to the surface of the work piece was 2.2 mm and the arc traverse 8.33 mm/sec.
• the hardened region consists of martensite and ferrite.
• a further advantage of the invention compared to the laser techniques is the amount of energy which is actually utilized in treating the work piece. For instance, in a typical laser application, only 300 watts of power is absorbed by a work piece from a 15 kilowatt carbon dioxide gas laser. In contrast, the TIG arc treatment method has an efficiency of utilization of electrical power of up to 30-50%.
• the techniques of the invention are especially suitable for the treatment of railway track and wheels of rolling stock. In the former case the arc producing apparatus can be mounted on a vehicle which runs along the track and hardens it.
• the invention is also very useful in hardening crankshafts, cylinder walls and valve seats of internal combustion engines.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Articles (AREA)
• Other Surface Treatments For Metallic Materials (AREA)

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Citations (12)

• 1980
  • 1980-05-06 WO PCT/AU1980/000009 patent/WO1980002434A1/en active Application Filing
  • 1980-05-06 GB GB8038466A patent/GB2059444A/en not_active Withdrawn
  • 1980-05-07 IT IT8021859A patent/IT8021859A0/it unknown

Patent Citations (12)

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