ABSTRACT: The composition of the work-piece to be processed affects the selection of bright annealing process. This paper mainly discusses the application of bright annealing process in stress-free annealing of brass work-pieces with high Surface finish requirements, the protective atmosphere and annealing temperature of bright annealed brass are analyzed. The results show that the annealed brass has good surface brightness, uniform mechanical properties and grain size. Keywords: Bright Annealing; Manganese Brass; stress relief annealing; Gas Protection Classification: Th 131.3; TH 161.14 Ref. ID: B

Foreword

Annealing is a common heat treatment process in mechanical processing. The aim of annealing is to soften the material after forging, welding or cutting, to improve its plasticity and reduce its hardness, to eliminate the internal stress and to refine the grains, a blank with good technological properties for subsequent processes. However, with the development of manufacturing technology and the continuous improvement of mechanical products, the ordinary annealing process can not meet the quality requirements of some parts. If the aircraft hydraulic pump on the slipper parts, its surface requirements in the stress relief annealing treatment has a very high finish; Ordinary annealing process can not meet such high finish requirements because of the formation of oxide layer on the surface of the parts. A clean surface can be obtained by the bright annealing process, and the wear resistance and fatigue strength of the parts can be improved. For most metals and alloys, the bright annealing process has been very mature. However, the bright annealing process is not perfect for brass with high content of zn-mn (easy to oxidize) .

1. Principle of bright annealing process for brass

Bright Annealing (also known as bright annealing) , because of the high cost of vacuum annealing, usually in the use of electric furnace (or gas fuel tube heating furnace) when the annealing into a certain composition of protective gas, the workpiece with harmful gas isolation, to avoid Surface finish compounds. The content of zn and MN in brass is high, so it is difficult to realize bright annealing of brass. At present, most of the use of purifying exothermic atmosphere for brass annealing, but generally can not achieve a bright effect. Especially for high zinc brass containing more than 15% zinc it is more difficult to achieve bright annealing. This problem has not been solved for a long time. The reason is that there are impurities such as residual O2, CO2, H2o and Co in the atmosphere of the furnace. Zinc and oxygen have a strong affinity, zinc and zinc alloys in the heating will be air, steam and carbon dioxide oxidation. In particular, there is a "dezincification" of brass. These are all factors affecting the bright annealing effect of brass [1,2] .

2mnMe+O2=2nMeO2/n(1)

nMe+CO2= mMen/mO1/m+CO (2)

nMe+H2O = mMen/mO1/m+H2(3)

nMe+CO = mMen/mO1/m+C (4)

Zinc in brass also redox with water vapor, Reaction Formula: Zn + H2o = Zno + H2o. In order to improve the above-mentioned situation, when the brass is treated by bright annealing, the reduction protective atmosphere (H 2/h 2O and CO/CO 2 ratio are large, H 2/h 2o and CO/CO 2 are more reductive components in furnace gas. It is different from the oxidizing component such as O2, CO2 in the furnace gas, the oxidizing protective atmosphere with low ratio of H2/H2O and CO/CO2 or the neutral protective atmosphere (between the reducing protective atmosphere and the oxidizing protective atmosphere) in order to insulate the oxidation of the workpiece surface such as O2, CO2.

In the reductive atmosphere, the reductive components such as H2 and Co in the protective gas isolate the oxidizing components such as O2 and CO2 in the air from the annealed workpiece and react with the metal oxides which may be formed, avoid the formation of oxide film on the surface of the workpiece, affecting the finish. The reduction reaction is as follows: MMEN/M + H2 = nMe + H2OMMEN/MO1/M + CO = nMe + CO2 according to the different alloy elements in brass, such as the content of zinc, as well as other elements and their content differences will have different requirements on protective atmosphere, temperature and other factors. According to the various grades of brass, the composition of protective gas will still be different in practical application. Table 1 shows the protective atmosphere used for bright annealing of some common brass.

2. For example, the slipper on the aircraft hydraulic pump (material: MANGANESE BRASS HMN60-3-1) was annealed with nitrogen and hydrogen mixture (N270% , H2≤30%) due to its high requirements on the Surface finish of the workpiece, in order to avoid the possible influence of CO, CO2, O2 on the surface quality of the workpiece in bright annealing. 

2.1 The experimental results of the annealing of slipper parts are shown in Table 2.

After the experiment, it was found that the surface of the workpiece was dark and stained with black.

Analysis of experimental results of 

2.2 slipper parts annealed

The darkening of the work-piece is due to the lower pressure of the protecting air, which can not completely remove the residual air and cause the material to oxidize with the oxygen (Formula 1) and Carbon Dioxide (formula 2) in the air during the heating process, oxide film is formed on the surface, which affects the finish. In addition, "Dezincification" (precipitation of the alloy element zinc in brass at a temperature of 450 °C) is likely to occur. The black stain on the surface is carbon black, which is formed by the reaction between material and Co (reaction Formula 4) . 3 in addition, the harmful phenomena such as crack and "hydrogen embrittlement" caused by metal hydrogen absorption in hydrogen-rich atmosphere should also be considered in the process of bright annealing. To sum up, the mistakes in the experiment basically come from the improper operation or mistake of the intermediate process.

2.3 bright annealing of slipper

Bright annealing of high zinc brass and copper alloys with high manganese content is easy to cause the problem of "Dezincification" , and zinc and oxygen have strong affinity, it is easy to react with Oxygen, water vapor and carbon dioxide, which may be mixed in protective gas, to form zinc oxide film, thus affecting the finish. Therefore, in order to effectively restrain the precipitation of zinc in brass, bright annealing can be carried out under the conditions of high pressure and rapid heating. Taking the bright annealing of HMn60-3-1 brass as an example, high pressure protective atmosphere (N270% , H2≤30%) was introduced to ensure the residual air to be exhausted, then, under the protective atmosphere of N270% and H2≤30% , the workpiece is heated rapidly to 480 °C by induction heating or electric contact heating, and cooled after holding for 90 Min, a smooth surface can be obtained. A smooth surface can be obtained by cooling at 480 °C for 90 min.

3. Conclusion

Compared with traditional annealing process, bright annealing can achieve high Surface finish, which has obvious effect on improving wear resistance and fatigue strength of annealed parts. For many high-precision parts, such as the slipper on the aviation hydraulic pump, the destress annealing of the workpiece is carried out by the bright annealing process in a reducing protective atmosphere, but the reducing protective atmosphere composition should be reasonably determined, to ensure that the annealing heat treatment, the workpiece still has a smooth surface. 

Source: Chinanews.com, by Lu Guangyu