Mechanical Properties of Forged Stainless Steel 321 Valves(Part One)

Abstract 
Nuclear valves often adopt forged stainless steel 321 as the valve body material in China. Stainless steel 321 forging billets manufactured by heating once and twice are introduced in this article. Mechanical properties of forged stainless steel 321 valves with the two processes are compared and the impact of the heat treatment on the mechanical properties of the two forging billets are further studied. Research shows that stainless steel 321 forging billets manufactured by heating once or twice and heat treatment at 1050℃ for 30 to 120 minutes can meet the national standard requirements for strength, elongation and section shrinkage; and compared with heating once, stainless steel 321 billet heating twice has smaller grain sizes and better yield strength. This study can provide technical reference for nuclear valve manufacturers when processing and purchasing stainless steel 321 blanks.
 
1. Overview
Nuclear-grade valves often adopt forged stainless steel 321 as the key valve body material in China because of its good and stable mechanical properties and good corrosion resistance. There are many types of nuclear-grade valves, the valve sizes vary greatly, and the required forging blank sizes also vary greatly. For large-size stainless steel 321 forging billets, the temperature in the forging process cools down slowly, and repeated thickening and drawing can be carried out without reheating, that is, the forging ratio and other requirements can be achieved in heating once. However, for small-sized forging blanks, the temperature drops quickly in the forging process. If the forging is not returned to the furnace for heating, forging in the low-temperature zone can easily cause the forging blank to crack. Therefore, the forging may be returned to the furnace multiple times to ensure the control of the forging temperature.
 

2. Experimental processes

According to the standard requirements for composition (GB/T 1220-2016), stainless steel 321 billet manufactured by a Chinese steel factory was selected. Its composition content is that C is 0. 048, Si 0. 45, Mn 1.09, S 0.001, P 0.022, Cr 17.80, Ni 9.30 and Ti 0.40. The entire bar was subjected to ultrasonic flaw detection and found that the product met the standard requirements and had no overall macroscopic defects.
 
Samples were cut from the blanks and subjected to one-heating and two-heating forging experiments. The one-heating forging process is that the holding temperature is 1100℃, the initial forging temperature not less than 1080℃ and final forging temperature greater than 850℃. Air cooling is performed after forging. The two-heating forging process is that the holding temperature is 1100℃, initial forging temperature not less than 1080℃, return temperature 1100℃, return time 30 minutes and final forging temperature greater than 850℃. Air cooling is performed after forging. The heat treatment was completed in a box-type experimental furnace with a heating rate of 10℃ per minute and a holding temperature of 1050℃. The forged blanks manufactured by different forging processes were kept at 1050℃ for different times (30 minutes, 60 minutes and 120 minutes) to complete the heat treatment, and then were water-cooled to room temperature. According to the standard (GB/T 228.1-2010), the tensile sample is processed; the length of the parallel section is 30mm, the gauge length 25 mm and the diameter of the parallel section 5 mm, M10 threads are processed on both ends and fixed with clamps. The tensile testing machine model is DNS100, and the tensile rate is 3. 6 mm per minute.

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