The quality control of modern enterprise is being diffused from the final test to the multi point, emphasizing the process oriented quality management, and the focus of management is moving forward continuously. Heat treatment is in the early and late stage of product parts manufacturing process, and its quality will have a great impact on the quality characteristics, matching production, productivity and reputation of the whole machine. Therefore, according to the new mode of quality management: quality planning, quality control and quality improvement, we need to analyze the factors that affect the quality of heat treatment process, so as to better control the quality of the process and produce products that are satisfied with users scientifically and economically.
1. The particularity of the production in the heat treatment workshop
The definition of special process in ISO9001:2008 standard is: "the output of process cannot be verified by subsequent monitoring or measurement, so that the problem will not appear until the product is used or after service delivery". The quality characteristics of heat treatment process can not be completely verified through inspection and test. After exposure, metallographic structure can not be determined by hardness test which is often used in production. In production, the hardness of the product is often qualified and the organization is unqualified, which causes the hidden trouble of quality or the decrease of the quality characteristics. In the process of heat treatment, the difficulty of the process is often increased because of the following reasons.
(1) only the inspection of process quality can not fully reflect the condition of process quality.
(2) the results of process quality tend to get feedback until the end of the process.
(3) the quality control is regarded as only the management and engineering personnel or the limitation of quality control.
(4) the feedback information scattered and incomplete, the channel is not smooth, the system is not perfect, it is difficult to take effective and timely control of the process.
2. The quality problems of heat treatment process
In the production process, the specific quality phenomena are usually shown in the aspects of heat treatment parts structure design, raw material quality control, quality control of technical documents, quality control of process materials, process execution, operation status of equipment, operator level and state. The quality analysis management system for heat treatment process is shown in Table 1.
(1) the quality of part design
Design quality is the starting point of quality control, which directly affects the quality characteristics, manufacturing difficulty and service life of the product. For heat treatment parts, the geometric shape, material selection, and no heat treatment defects (such as deformation, cracking, etc.) must be considered during the design process. It is necessary to improve the design quality of heat treatment process, take full account of the difficulty of heat treatment, cost cost and production cycle.
(2) raw materials
The parts are made of all kinds of steel. If there are various defects and problems in the steel itself, it may be expanded during heat treatment, or it will cause heat treatment of defects and problems, which will affect the quality of heat treatment and even scrap parts.
(3) forging quality
A considerable number of heat treatment parts are processed by forging and then processed into the heat treatment process. The purpose of forging is not only to make parts rough, but also to transform carbide distribution and improve the quality of internal tissues through forging. The defects in the steel after forging will have a great influence on the quality of heat treatment, such as the forging crack, the unevenness of the tissue, the over burning and the appearance defects.
(4) mechanical processing quality
The surface quality of the machined parts has an important influence on the heat treatment performance of the part. The residual stress caused by mechanical processing may cause deformation or waste in the heat treatment of parts. Rough surfaces, scratches, and knife lines all lead to stress concentration, resulting in the formation of heat treatment cracks.
(5) heat treatment furnace
The heat treatment process is achieved by a heat treatment furnace. The heating quality of parts is mainly determined by the heat treatment furnace, namely the design rationality, sealing performance, heat preservation, furnace temperature uniformity, furnace components, furnace temperature control, atmosphere control and so on.
(6) heat treatment heating quality
By changing the internal structure of materials by heat treatment, the parts must be put into a heat treatment furnace and heated to a certain temperature for austenitizing. The heating quality has a direct effect on the performance of the parts. Mainly through heating temperature, heating speed and holding time to control the number and uniformity of body tissue composition, carbide morphology and grain size. There should be no defects such as under heat, overheating, oxidation, decarbonization and so on to avoid hot cracks. Prevent parts from improper placement, heating temperature uneven and cause parts deformation and so on.
(7) the cooling quality of heat treatment
The cooling quality of the parts after austenitizing is the key to improve the quality of heat treatment of parts. The structure state of the parts after cooling, the type of stress and the size of the stress have a great influence on the performance of the parts. According to the material and size of the parts, the cooling medium, cooling speed and cooling method should be chosen correctly to obtain the due cooling effect. To avoid defects such as quenching crack, deformation, uneven hardness, incomplete quenching and so on.
(8) the quality after heat treatment
The auxiliary treatment and defect prevention after heat treatment are also an important aspect of the quality control of heat treatment. The various parts after heat treatment, the surface may be dirt scale, residual salt, oil, not only affect the appearance, will cause corrosion and damage to the parts must be clean. It is also necessary to straighten, correct and prevent rust, and to prevent the occurrence of grinding cracks, EDM cracks, mechanical damage and other defects.
Quality control and analysis of heat treatment process
Table 1 quality analysis management system for heat treatment process
heat treatment