I. INTRODUCTION Stainless steel has a wide range of features such as corrosion resistance. With the improvement of its mining and metallurgy technology and production capacity, its application field is expanding. The powder metallurgy stainless steel products have less cutting and processing, have higher precision than precision castings, and do not have the advantages of environmental protection issues such as electroplating and other coating products, and have achieved rapid development in the past ten years. Such as car windshield rearview mirror base, foreign countries have long used stainless steel parts instead of colored castings or other anti-rust coating parts.

Fuzhou Fuqing Fuyao Glass Group Co., Ltd. is one of the largest automotive windshield manufacturing bases in the world today. In order to meet the needs of the export market, a large number of imported powder metallurgy stainless steel mirror bases are required to be imported every year. Due to the constraints of availability, cycle, and price, it is necessary to localize it. With the participation of relevant parties in mid-2001, the agreement established a special production base. After continuous efforts, localization was successfully developed at the end of 2002 and the production was put into batch production the following year. In 2005, the output was about 1 million, and there were some exports. The expanded production line now has an annual capacity of 3 million pieces.

Second, process equipment and raw material selection According to the investigation of a small number of domestic powder metallurgy stainless steel products factory, basically using vacuum sintering process. Taking into account the large one-time investment in vacuum sintering and may not be convenient for stable, continuous, large-volume production, etc., according to the proposal of the technical partner, it was decided to use ammonia decomposition atmosphere and continuous furnace sintering design. The main equipments selected are: (1) 100 tons of automatic powder metallurgy hydraulic press; (2) high-temperature through-type molybdenum wire furnace, furnace tube size: length, width and height is 4000×150×130mm; (3) 5M3/h is low Dew point (≤ 60 °C) ammonia decomposition device; (4) other related before and after processing equipment and testing equipment.

AISI 430L or 434 stainless steel materials are selected according to the relevant documents. Trial production has used the 430L water-atomized powder produced in Hebei and Zhejiang respectively for testing.

Third, the development of the research and development of the first phase of technical key points. Based on sporadic reports of foreign industrial production, self-reliant exploratory experiments of various programs were conducted. Including changes in sintering temperature, holding time, loading amount and method, adding sintering activator and other various options. After several months of trial and error, regardless of how the program is combined and optimized, the sample after sintering is not able to see a little bit of stainless steel luster, and the strength, toughness, hardness, and fracture metallographic structure of the sample are compared with those of the imported sample. The difference is far away.

Fourth, parts corrosion resistance performance test (1) Selection of test methods:

In fact, all metals or alloys are difficult to guarantee that they will never rust. Whether they produce corrosion depends on the conditions of their working environment. However, due to the combination of work environment conditions of parts and their own factors, there are many combinations and changes. In the face of this extremely intricate situation, it is obviously unrealistic to formulate a universal test method to determine the corrosion resistance of materials. Can only be based on actual needs and possible formulation or selection of a test method. The commonly available test methods are:

The first is the actual use test. It takes a long time (some years up), costly, and difficult to obtain quantitative results.

The second is the laboratory test. It is divided into analog test and accelerated test. The former is to artificially simulate various media and conditions formed in the natural world as accurately as possible in a small simulation load. The accelerated test is to strengthen one or a few control factors in the actual conditions, so that it can be easily judged within a short time. Because it is convenient to freely choose and strictly control each influencing factor, it can be accurately measured, the time of the test is short, and the result is reproducible, etc., and there are many methods to formulate corresponding national or industry standards. Such as "artificial atmosphere - salt spray corrosion test" and "hot and humid corrosion test" and so on belong to this category. This part is selected "hot and humid corrosion test" method for assessment.

(2) Test results and analysis:

The test environment atmosphere system was selected to be in a sealed container at 60±2°C and a relative humidity of 95±4%, and the corrosion condition after 48 hours of storage was recorded. Since the relevant documents (including the powder metallurgy stainless steel material standard MPIF 35) do not provide specific judgment criteria, the parties agreed to take a comparative test of the sample of the trial part and the imported part. That is to say, several imported parts and trial parts are to be taken at the same time under the same condition. As a result, it was found that the occurrence of rust spots in both cases was of various lengths (that is, some parts had "point corrosion" locally), but they were almost the same. Repeated tests thereafter showed better re-dominance.

Through the above assessment and acceptance, it shows that the products developed have fully met the requirements of imported parts. Therefore, both parties unanimously determined that the development was fully successful, and they put into mass production the following year.

V. Conclusion:

Through the research work on the localization of stainless steel mirror bases for automobiles, it was shown that the ammonia decomposition atmosphere and molybdenum wire electric furnaces and other equipment must meet the requirements for producing powder metallurgy stainless steel parts. The key is to take the necessary hard and soft technical measures. The dew point of the furnace atmosphere is always lower than the "safety value" required for continuous sinter reduction.

In order to search for the cause of the failure, the quality indicators of the purchased equipment were reviewed, and the results were all up to standard (such as ammonia decomposing atmosphere dew point reached ≤ 60 °C, etc.). It seems that they all seem to be able to meet the requirements of major process parameters reported abroad. But why can't the sintered sample be qualified? But there are its special difficulties and "technical decision"? After trying hard to find and test again, I haven’t found the answer for a long time. In the absence of a start, the research team was automatically dissolved and the localization work was suspended.

The second stage trial production. The newly formed research team summarized lessons learned from previous failures and based on previous experience in innovation and industrialization research and development, conducted domestic and international collections of scientific literature and related “living information” through careful digestion, absorption, and induction analysis. On the basis of a clearer understanding of the difficulties of stainless steel sintering, finally solved the solution. It is briefly described as follows:

(1) Difficulties in stainless steel sintering and necessary and sufficient conditions for successful sintering. Stainless steel is based on Fe--Cr alloy. According to the Tamman's law, Fe--Cr alloy ferro-anode potential can produce leapfrogging only when Cr content exceeds 12.5% ​​(atomic content; weight is 11.7%). The improvement of the anti-corrosion properties of the alloy is improved by the improvement of the alloy, thereby ensuring that it does not produce rust in certain media conditions. The reason for this anti-corrosion mechanism is that Cr element is very easily deactivated. The so-called passivation means that Cr reacts with the surrounding medium to form an extremely thin (a few nm) oxide film and a coating phenomenon occurs. Due to the loss of chemical activity, the passivation layer exhibits properties similar to those of inert metals, and is dense under certain conditions, and is not susceptible to dissolution or the like. Even if it is damaged, it will be passivated again.

As mentioned above, it is not difficult to find that it is the presence of such a passivation layer in the surface layer of the stainless steel powder, which greatly hinders the progress of its sinter reduction, thereby making the powder metallurgy stainless steel sintering very difficult.

In order to overcome this difficulty, the law of oxidation-reduction reaction of chromium oxide in hydrogen was studied. From the related reaction diagram, we can see that pure Fe--Cr stainless steel does not seem to be particularly harsh (such as the dew point (critical value) of the atmosphere required at different sintering temperatures (such as about -23 °C at 1150 °C ). But this is just the dew point of the atmosphere necessary for the chromium oxide to be reduced.

Industrial practice shows that in order to fully ensure the continuous successful sintering of stainless steel powder compaction, more factors must be considered. For example, no pollution occurs during the sintering and cooling process and the alloy contains other components, generally requiring a lower atmospheric dew point. There should also be a certain safety factor and so on. Therefore, in order to ensure the normal operation of stainless steel sintering, not only the dew point of the decomposed ammonia gas to be introduced should be dried to -60 °C, but more importantly, how to discharge the water vapor generated by the redox reaction in time. In order to always keep the dew point of the furnace atmosphere within the required specifications.

(2) The main technical measures. According to the above, through trial and error, a number of technical measures have been taken to ensure that the original equipment is in the entire sintering process, ensuring that the "furnace atmosphere dew" meets the requirements of stainless steel sintering and thus achieves successful sintering. The main points of its technical measures are:

● Powder raw materials require low carbon content and oxygen content; all the related items with sintering are required to maintain high dryness;

● Reform the structure of the existing sintering furnace. Such as adding pre-sintering section, changing the size of the furnace tube and adjusting the flow and flow of reducing gas;

● Change the ammonia decomposition device and strictly select anhydrous liquid ammonia and change the drying cycle process specification;

● Use a higher sintering temperature (such as 1250-1300 °C) and properly control the productivity to reduce the dew point of the atmosphere.

The third stage of development is to further solve problems such as dimensional accuracy, mechanical and physical properties, and appearance compliance (all imported parts are used as standards). Because the piece is in the form of a sheet, and the steps, the taper and the taper have certain accuracy requirements, and the high-temperature sintering shrinks greatly, so the design of the size and weight of the preform must be adjusted several times. . In addition, after continuous trials, it also explored the best post-processing equipment and processes, making the surface finish of other parts meet the standards of imported parts.