Study on industrial heat treatment process of 304 stainless steel

In order to realize the bright heat treatment of 304 stainless steel evaporator, a protective atmosphere is applied to the small mesh belt heating furnace. Through the combination test of several groups of process parameters, the hydrogen flow rate is 4.5-5.0 m3/h and the pressure is 0.2 MPa; Heat treatment temperature 1060 ± 10 ℃; Holding time 6.5 min; The process parameter of mesh belt speed is 180mm/min. The influence of heating temperature and heating time on the structure and performance of evaporator was discussed.

Evaporator is an important component of the four major components of refrigeration, low-temperature condensing liquid through the evaporator, and the outside air for heat exchange, gasification and heat absorption, to achieve the effect of refrigeration. Currently, the refrigeration industry are using white copper evaporator tube, but, considering the heavy metal ion copper to the human body, international, for the evaporator closely related to the human diet, many companies began to study the use of stainless steel instead of copper alloy. Bright heat treatment process has an important impact on the performance of stainless steel evaporator.

1. Test materials and methods

1.1 Test material

304 stainless steel is one of the most widely used austenitic stainless steel, because of its good plasticity (elongation ≥ 40%), very suitable for stamping, bending and other processing methods. The evaporator is a tubular, multiple bending characteristics of the product, see Figure 1, the forming performance of the material has higher requirements, therefore, with 304 stainless steel as an alternative material for the evaporator. This experiment uses food-grade 0.5mm wall thickness of 304 stainless steel as the evaporator processing with materials.
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Figure.1 evaporator products in kind

1.2 Test equipment

General stainless steel to do bright heat treatment, the use of vacuum heat treatment or reduction protective atmosphere heat treatment, taking into account the application of mass production, the equipment used in this experiment is a small mesh belt heating furnace. The protective atmosphere of the furnace consists of ammonia decomposition of nitrogen and hydrogen, the ratio can be artificially controlled. Among them, hydrogen is the reducing atmosphere for furnace production to combine with oxygen inside the furnace; nitrogen is used as the purging gas for the web belt path to purge the air or hydrogen inside the furnace during furnace ignition, furnace shutdown or sudden accidents, and to purge and seal the inlet and outlet of the web belt path during normal production.
Cold room using cooling water jacket cooling muffle tank, and the use of nitrogen and hydrogen gas mixture forced cooling to ensure that the cooling rate of stainless steel after heat treatment.

1.3 Bright heat treatment process

Bright heat treatment of 304 stainless steel involves two parts: solution treatment and bright treatment.

(1) Solution treatment process research. In order to study the effect of solid solution treatment process on the organization of stainless steel condensate tube, the two factors of heating temperature and heating time were selected, the process curve and process parameters are shown in Figure 2, Table 1, where the heating time relies on adjusting the speed of the mesh belt control, after the end of heating, the condensate tube cooled sharply to below 350 ℃ at a cooling rate of about 55 ℃ / s.

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Figure.2 Solution heat treatment process curve diagram
Table.1 Solution heat treatment process parameters

Process parameters  Parameter setting
Heating temperature,  100010201040106010801100
Heating time, min 36.510

(2) Brightening process research. In order to study the impact of the protective atmosphere on the surface of the stainless steel condensate tube in the brightening process, the hydrogen flow rate factor was chosen to influence the surface brightness of the stainless steel condensate tube. The hydrogen flow rate is controlled according to Table 2, and the ratio of the gas mixture is controlled to ensure constant pressure.
Table.2 Hydrogen flow rate process parameters

Process parameters  Parameter setting
Hydrogen flow, m3/h 34.56

2. Experimental results and discussion

2.1 The effect of heating temperature and heating time on the organizational properties of stainless steel condensers

The purpose of solid solution treatment of the condenser is to remove the stress of the workpiece in the forming process and eliminate the effects of carbon segregation brought about by cold working, welding, etc. This requires the heating process, all stainless steel into austenite, and after rapid cooling, to obtain a single-phase austenite organization, to avoid the precipitation of Cr23C6-based carbides at the grain boundaries, causing material sensitization, reducing the corrosion resistance of the workpiece.

2.1.1 Heating temperature

Under the heating temperature of 1000℃, the organization is austenite, a small amount of ferrite and a small amount of carbide, due to the low solid solution temperature, carbide is not all dissolved, solid solution is not sufficient. When the heating temperature is 1020℃, there is still a small amount of ferrite and carbide not dissolved into austenite, but the grain boundary with polygon shape is clearly visible, which is due to the recrystallization of austenite during the heating process. Heating temperature of 1040 ℃, carbide has all dissolved, can obviously find the grain and grain boundary line, and austenite organization is uniform and neat, but there are still a small amount of ferrite organization. When the heating temperature is 1060℃, the metallographic organization is more uniform than the heating temperature of 1040℃, the grain boundary line is more obvious, and there is only a small amount of lamellar ferrite organization. When the heating temperature is 1080℃ and 1100℃, the uniformity of the organization is further improved, however, the grain is significantly coarser than the heating temperature of 1060℃, and there is a small amount of ferrite and carbide at 1100℃.

2.1.2 Heating time

When the heating time is 3min, the grain boundary line can be distinguished, and the organization is not dissolved as a single phase, with ferrite and a small amount of carbide. When the heating time is 6.5min, the grain boundary line is more obvious, and there is only a trace of lamellar ferrite organization. However, when the heating time was 10min, the ferrite and carbide started to increase again.
In summary, after the workpiece was treated with a heating temperature of 1060℃ and a heating time of 6.5min, the ferrite and carbide were almost completely dissolved into the austenite, which had a more desirable grain size and better tissue consistency compared with other heating temperatures.

2.2 The effect of hydrogen flow rate on the surface of stainless steel condenser

The pressure of hydrogen is kept constant at 0.2 MPa, and the flow rate of the incoming is 3 m3 / h, 4.5 m3 / h, 6 m3 / h, the surface is shown in Figure 3.

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Figure.3 surface of 304 stainless steel under different hydrogen flow rates
In the flow rate of hydrogen is 3.0m3/h respectively, the surface of stainless steel is dark green, indicating that the surface of stainless steel has been oxidized. In the hydrogen flow rate increased to 4.5m3 / h, the stainless steel surface is bright, not oxidized, indicating that the oxygen in the furnace has been fully reduced by hydrogen. In the flow rate of hydrogen gas increased to 5.0m3 / h, the surface of stainless steel is also presented bright. In summary, the pressure remains unchanged, in the case of hydrogen flow increased to 4.5m3 / h, the stainless steel surface bright, taking into account economic factors, hydrogen flow control in 4.5 – 5.0m3 / h more reasonable.

2.3 Discussion

2.3.1 Heating temperature

The decomposition of chromium carbide solid solution, and heating temperature has a fairly important relationship, in the range of 1000 – 1080 ℃, with the increase in heating temperature, carbide from partial solid solution to basically all solid solution. However, in 1060 – 1100 ℃, with the increase in heating temperature, austenite grains began to coarsen, and a small amount of carbide was found. It is known that the increase in heating temperature, can improve the solid solution of carbon in austenite, there are data show that 304 stainless steel at 600 ℃, the solid solution of carbon for 0.02%, 1000 ℃ for 0.18%, 1200 ℃ for 0.34%, but why at 1100 ℃, the metallographic display and carbide appear? I think, in the austenite grain coarse, the grain boundary of the indicated area will be reduced, and therefore in the grain boundary, the carbide precipitation density will increase, compared with 1060 ℃, carbide precipitation time to shorter, earlier.

2.3.2 Heating time

Although the solid solution of carbon reaches 0.18% at 1000°C and above, it still takes some time to ensure that all the carbon is dissolved. Of course, with the increase of time, austenite will also appear coarsening, so in the case of 1060℃×10min process, the microstructure also found precipitated carbide.

2.3.3 Protective atmosphere

Hydrogen as a protective atmosphere can protect the stainless steel condenser to avoid oxidation, decarburization, and hydrogen is a reducing atmosphere, can reduce the surface of the condenser oxide film, so that the surface of the condenser more bright. The amount of hydrogen gas into the furnace will directly affect the atmosphere, if the furnace is not filled with hydrogen, the surface of the condenser is very easy to oxidize at high temperatures and appear green.

3. Conclusion

  • (1) 304 stainless steel condenser best heat treatment process parameters are: heating temperature 1060 ± 10 ℃; heating time 6.5min; mesh belt speed of 180mm / min;
  • (2) 304 austenitic stainless steel should be avoided for a long time or heating above 1080°C to prevent carbide precipitation.
  • (3) hydrogen flow in the range of 4.5 – 5.0m3 / h, pressure of 0.2MPa, stainless steel condenser surface without oxidation, the surface is bright.

SourceChina Pipe Fittings Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)

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