## 1 Introduction
### Low-Temperature Chromizing Process  
The treatment temperature of the traditional chromizing method is generally higher than 1000 \({}^{\circ}\)C for about 6 h. In the process of long-time high-temperature chromizing, the grain of the infiltrated workpiece often grows, resulting in the decline of the mechanical properties of the infiltrated workpiece, which limits the wide application of chromizing technology. This paper adopts the method of low-temperature salt bath chromizing treatment to realize the chromizing treatment of 304 stainless steel at 700 \({}^{\circ}\)C. The specific process includes the following steps:  
* Sodium salt or potassium salt is selected as the primary solvent, and low melting point chromium salt is used as the auxiliary agent.  
* Chromium powder or ferrochromium powder is used as the chromizing agent.
* The workpiece to be infiltrated, one of sodium salt or potassium salt, chromium salt and chromium powder or ferrochromium powder infiltrating agent are put in a closed container.
* The temperature is raised to a specific temperature which is kept for a particular time.
* Hydrogen is continuously injected into the closed container under a certain pressure during the insulation process.  
The solvents are NaCl and CrCl\({}_{3}\cdot 6\)H\({}_{2}\)O, where the weight proportion of CrCl\({}_{3}\cdot 6\)H\({}_{2}\)O is 25%. Chromium powder is added, and the weight ratio of chromium powder to CrCl\({}_{3}\cdot 6\)H\({}_{2}\)O is 1:6. Those materials are mixed and then poured into a crucible and loaded with the 304 stainless steel parts. The crucible is placed into a closed container with a vacuum and starts to heat up. After the temperature rises to 620 \({}^{\circ}\)C, hydrogen is slowly injected into the container. When the hydrogen pressure is higher than 0.02 MPa, injection of hydrogen is stopped, and the temperature is kept for 4 h. During the insulation process, if the pressure is lower than 0.02 MPa, the hydrogen is continuously injected until the pressure is higher than 0.02 MPa. It is then cooled to within 100 \({}^{\circ}\)C and taken out the workpiece. The principle of chromizing process is shown in Figure 1. The chromizing temperature curve is shown in Figure 2. After taking out the workpiece, dehydrogenation treatment is conducted at 200 \({}^{\circ}\)C for 4 h. The obtained chromizing layer is 10-20 \(\upmu\)m, as shown in Figure 3.  
Figure 1: Principle diagram of Solid Power Chromizing process.  
Figure 2: Chromizing temperature curve.  
In the above processes, the melting point of NaCl is 801 \({}^{\circ}\)C, and the melting point of CrCl\({}_{3}\) - 6H\({}_{2}\)O is 83 \({}^{\circ}\)C. After the two are mixed in a certain proportion, the melting point of the mixture can be controlled below 650 \({}^{\circ}\)C. Under the condition of hydrogen, the following reactions are occurred by  
\[2\text{CrCl}_{3}+\text{H}_{2}=2\text{CrCl}_{2}+2\text{HCl}, \tag{1}\]  
\[\text{CrCl}_{2}+\text{Fe}=\text{FeCl}_{2}+[\text{Cr}], \tag{2}\]  
\[\text{CrCl}_{2}+\text{H}_{2}=2\text{HCl}+[\text{Cr}]. \tag{3}\]  
Thus, it can be observed that the active chromium atoms produced by Reactions (2) and (3) penetrate into the surface of 304 stainless steel, and the thickness of the chromized layer is also increased with the reaction progress.