In general, heat exchanger tube, fin, flange or body panel composing heat exchanger in instantaneous water heater, hot water supply system, air conditioner, refrigerating machine etc. are produced by using pipe materials (seamless copper alloy tube) or plate made from superior heat resisting phosphorus deoxidized copper (JIS C1220). At the time of the production, the phosphor copper solder (JIS Z3264 BCuP-2) etc. are used as brazing materials, and to braze the heat exchanger tubes, it is necessary for such tubes to be heated to about 800° C. because the melting point of phosphor copper solder is 710° C. in solidus temperature and 795° C. in liquidus temperature.
However, in the case that the heat exchanger tubes made of phosphorus deoxidized copper are locally or wholly heated at as high as 800° C. by brazing, crystal grains of phosphorus deoxidized copper become large. In addition, the matrix strength of phosphorus deoxidized copper is low by its nature, so that the mechanical intensity (for example, tensile strength, proof stress, elongation, fatigue strength, hardness etc.) after heating (after brazing or welding) becomes low remarkably. Although such lowering degradation of the mechanical intensity due to the grain growth in phosphorous deoxidizing copper appears at different levels according to the production conditions where the material for heat exchanger tube etc. is prepared. Such degradation can be recognized remarkably when the material is heated at temperatures higher than 600-700° C. in general. Therefore, in the case of heat exchangers equipped in instantaneous water heater and hot water supply system using heat exchanger tubes made of phosphorus deoxidized copper, there was a problem in the durability since the mechanical intensity of such heat exchanger tubes lowers at the production stage. By way of example, since the heat exchanger tubes in instantaneous water heater, hot water supply system and air conditioner etc., repeat thermal expansion and thermal contraction in turn while in use. Therefore, there is a possibility that the heat exchanger tubes are locally destroyed by fatigue due to the repeated loading, and consequently, the life cycle of the products become short. This problem of durability shows that the phosphorus deoxidized copper cannot be used for the heat exchanger tubes, because the low durability is especially remarkable in the heat exchanger tubes using any heat medium gas aside from HCFC system fluorocarbon. Recently, in order to prevent the discharge of green house gas effect and depletion of ozone layers, CO2 and HFC system fluorocarbon tend to be used instead of conventional HCFC system fluorocarbon as the heat medium gas for heat exchangers equipped in hot water supply system and air conditioner etc. When such CO2 or HFC system fluorocarbon is used as the heat medium, however, the condensing pressure needs to be higher comparing to the case of HCFC system fluorocarbon. Therefore, since the high condensing pressure acts on the heat exchanger tubes of the heat exchanger using heat medium gas aside from HCFC system fluorocarbon in a cycle, proof pressure, namely proof stress of heat exchanger tubes (0.2% yield strength) or yield stress is not high enough. As a result, there is a possibility to produce cracks in the heat exchanger tubes and to cause dimensional change of the heat exchanger tubes by long term use, in a case that the phosphorus deoxidized copper is used for the heat exchanger tube. The same problem occurs in the plate materials used for body panels. For example, in the case of body panels used for heat exchangers of hot water supply system and water heater etc., the local fatigue fracture caused by the repeated stress by the expansion in use and the contraction when not in use may lead to a possible fire accident. In addition, as to the fins installed in heat exchanger tubes of hot water supply system and air conditioner etc., the mechanical intensity (proof stress in particular) lowers remarkably at the production stage, so that the fins may be easily deformed by small external force (by a slight touch of hands or tools during maintenance or cleaning). Thus the thermal efficiency of the heat exchanger will be lost greatly. Furthermore, in the case of the brazed parts (water supply pipes, hot water supply pipes, gas piping, electric apparatus plumbing and chemical machinery plumbing etc.) made of phosphorus deoxidized copper, it is required that the wall thickness must be larger than necessary because of the remarkable lowering degradation of strength after brazing. Also there is a possibility of various kinds of trouble (water leakage, malfunction etc.) originating from the deformation due to small external forces in use and/or during maintenance. In addition, by the grain growth, susceptibility to stress corrosion crack, pitting corrosion and localized corrosion becomes high, while general corrosion resistance becomes low, resulting in many problems when in use.