How to increase the mechanical & corrosion resistance of sintered parts

Introduction

For iron-based powder metallurgy materials, which due to their structures contain certain pores, compared with dense materials with the same composition, they have the characteristics of lower hardness, poor corrosion resistance and air tightness.
After steam treatment, they will A hard and dense oxide layer composed mainly of Fe3O4 that is firmly bonded to the iron matrix is generated on its surface, which can greatly improve the hardness, compressive strength, rust resistance, wear resistance and gas resistance of the sintered iron-based material.
When used for sealing treatment, the cost is only about 15% of copper infiltration and 30% of plastic impregnation, which is relatively economical, so it has been widely used in the field of iron-based powder metallurgy.
It is most commonly used in parts with 5.4~7.0g/cm3 (68~89% theoretical density).
For powder metallurgy parts with a density higher than 7.0g/cm3, it is generally believed that it can only improve the corrosion resistance of the parts and has little impact on the mechanical properties.

Experimental methods

Since in production, customers have many requirements for the hardness and air tightness of parts after steam treatment, hardness is used as the main evaluation index here.
The specific method is: continuously extract 16 samples (numbered 1 to 16 respectively) in batch production, and sinter them in the same furnace at the same time (sintering temperature 1120°C, time 30 minutes, density after sintering 6.9±0.03g/cm3), weighing Heavy, the parts are divided into 4 groups, as shown in Table 1., and the treatment processes are shown in Figure 1.
The steam pressure during the treatment process is maintained between 30±4KPa. After steam treatment, the weight gain rate and hardness of these samples were compared horizontally and vertically, and the influence of shaping and vibration deburring on the steam treatment effect was analyzed.

Table 1. Testing Groups

Test Group	Sample Batch	Sample Type	Steam Treatment
A	#1,#2,#3,#4	Sintered parts	Two-stage
B	#9,#10,#11,#12	Shaping, vibration deburring parts	Two-stage
C	#5,#6,#7,#8	Sintered parts	One-stage
D	#13,#14,#15,#16	Shaping, vibration deburring parts	One-stage

Steam Treatment Process Curve

Figure 1.

Testing Analysis & Result

Table 2 shows the relevant data after steam treatment in four groups of experiments A, B, C, and D. It can be seen from it that whether the one-stage or two-stage steam treatment process is used, the weight gain rate of the sintered parts after steam treatment is higher than that of the Shaping, vibration deburring parts.
The main reason for the large size of vibration deburring parts is that some of the open pores will be closed or refined after shaping and vibration deburring.
In this way, in addition to the closed pores during steam treatment, those that have been refined the pores will also be closed relatively quickly, thus hindering the reaction from proceeding to deeper parts, and ultimately the weight gain rate will be smaller than that of sintered parts.

For samples that use a two-stage steam treatment process, since they are first steam treated at 500°C for 1.5 hours and then raised to 560°C for treatment, the reaction speed at 500°C is much gentler than that at 560°C, even for samples with sealing effect such as shaping and vibrating parts, the steam can have sufficient time to react with it at this temperature without clogging the pores, so that the sintered parts and the shaping and vibrating parts have a large weight gain rate and the difference is not big.

For samples treated with one stage of steam, since the steam treatment is carried out at 560°C, the reaction speed is faster, so that the refined pores in the shaping and vibrating parts are quickly closed, so that the reaction only occurs when the samples is relatively small. It is carried out in shallow parts, and the generated oxide film is very thin. Compared with sintered parts, because the pores of sintered parts are larger, even if the reaction is faster, the pores will not be closed in a short time, and the reaction can still proceed in deeper parts. In this way, after a period of steam treatment, the weight gain rate and hardness of the sintered parts are obviously much greater than those of the shaping and vibrating parts.

Conclusion

For the same kind of samples, the weight gain rate of two-stage heating steam treatment is greater than that of one-stage heating treatment, and the corresponding hardness is also higher.

For sintered parts, the weight gain rate and hardness are not much different when using one-stage heating and two-stage heating for steam treatment. However, for samples that have sealing effects such as shaping, vibration deburring, and grinding after sintering, Generally speaking, the weight gain rate and hardness of the two-stage heating treatment are significantly greater than that of the one-stage heating treatment. Therefore, for those samples that have hole sealing treatment after sintering when the requirements for hardness, air tightness, etc. are high, it is better to adopt a two-stage heating treatment method.