TP321 Stainless Steel Seamless Pipe
The factors that affect the intergranular corrosion resistance of TP321 stainless steel seamless steel pipe are analyzed, and the reasons that may lead to the reduction of corrosion resistance are studied from the aspects of chemical composition , metallographic structure , process flow , heat treatment system, etc., and measures are taken to eliminate the adverse effects . Through a series of improvement measures such as controlling the chemical composition , adjusting the heat treatment system, and optimizing the degreasing process, the pass rate of the intergranular corrosion test of TP321 stainless steel seamless steel pipe was stabilized at a level above 95% .
Key words: seamless steel pipe; TP321 stainless steel; intergranular corrosion; chemical composition;
Abstract： Analyzed here in the article are the factors affecting intergranular corrosion resistance of the TP321 seamless stainless steel pipe， involving such aspects as chemical composition， metallographic structure， process flow， heat treatment schedule ， etc. Accordingly， relevant actions are taken to eliminate the negative effects. Thanks to implementation of improvement actions like chemical composition control， adjusting heat treatment schedule ， and optimizing the degreasing process， etc.， the primary qualification rate of intergranular corrosion test to the TP321 seamless steel pipes has been kept at the level above 95%.
Key words : seamless steel pipe ; TP321 stainless steel ; intergranular corrosion ; chemical composition ; heat treatment ; degreasing
Intergranular corrosion of stainless steel is a corrosion damage phenomenon, which is manifested by the loss of bonding force between grains, resulting in the deterioration of the strength of the material. There are many different theories for the cause of intergranular corrosion, such as chromium-depleted theory, grain boundary adsorption theory, precipitate phase metastable theory, metastable phase dissolution theory, stress theory, precipitate phase morphology theory, and corrosion electrochemical theory.
Chromium-depleted theory is the earliest and widely accepted theory . For 18-8 type austenitic stainless steel, the crystal lattice at the grain boundary is incomplete, which is conducive to the diffusion of metal atoms; Cr at the grain boundary and its adjacent areas will be due to the precipitation of carbide Cr 23 C 6 at the grain boundary The impoverishment phenomenon occurs, resulting in a chromium-depleted area around the grain boundary. When the Cr mass fraction is reduced to about 12 %, corrosion occurs along the grain boundary of the material in some corrosive media , and the bonding force between the grains is lost, that is, Intergranular corrosion phenomenon .
TP321 stainless steel ( UNS S32168 , 06Cr-18Ni11T in GB/T 20878-2007 "Stainless Steel and Heat-resistant Steel Grades and Chemical Composition" ) is based on TP304 stainless steel by adding Ti element to enhance its resistance to intergranular corrosion and resistance to corrosion. High temperature performance, the principle is to form stable MC carbide TiC , measured by the free enthalpy change of carbide formation, TiC is far more stable than chromium carbide, which can reduce the formation of chromium carbide.
In markets in developed regions such as Europe and the United States, TP321 stainless steel seamless steel pipes have been gradually replaced by low-carbon and ultra-low-carbon stainless steel seamless steel pipes such as TP304L and TP316L ; but in China, the demand for TP321 stainless steel seamless steel pipes is still very large. According to the latest data released by the forum dssF , the apparent consumption of TP321 stainless steel seamless steel pipes in China in 2012 was around 100,000 tons . Due to the limitations of production technology and inspection conditions, the pass rate of intergranular corrosion resistance of domestically produced TP321 stainless steel seamless pipes is not high, and the pass rate of one inspection is about 80% . Shanxi Taigang Stainless Steel Pipe Co., Ltd. (abbreviated as Taigang Stainless Steel) has produced TP321 stainless steel seamless steel pipes accounting for about 35% of the total production since it was put into production in 2009. The pass rate of intergranular corrosion inspection is about 70% , which is far lower than that of other stainless steel pipes. The level of the product ( M 95%) . In order to improve the finished product rate and better meet the needs of users, technical research has been carried out on the intergranular corrosion resistance of TP321 stainless steel seamless steel pipes . The level of foreign advanced manufacturers such as Sumitomo Metal Corporation (SUMITOMO) .
1 Analysis of intergranular corrosion principle of TP321 stainless steel seamless steel pipe
1.2.1 Influence of alloying elements
From the chemical formula of TiC , it can be seen that under the condition that the content ratio of Ti and C is 3.99 , Ti in steel can make all C be combined and fixed. But in fact, due to the influence of factors such as chemical equilibrium and heat treatment conditions, pure theoretical calculations cannot meet the requirements of all fixed C. According to the research results of literature, the Ti content required to avoid intergranular corrosion should be:
w A 4w ( C +3.42w < N ) -0.001
( 1 )
the research results of the University of Complutence in Spain , for TP321 steel, reducing the C content to less than 0.03% will reduce the intergranular corrosion susceptibility of stainless steel, and its effect is better than that of increasing the Ti content.
Cr and C form Cr 23 C 6 , according to the reaction formula 23Cr+6C =CdC 6 , the following equilibrium relationship should be satisfied:
K=1/ [( r Cr X Cr 23 ( r C X C 6 ]
( 2 )
In the formula, K — — — equilibrium constant;
r Cr , r C — — — Activity coefficients of Cr and C , carbide
The activity coefficient is 1 ;
Xg 、 Xc _ — Mass fractions of Cr and C in equilibrium .
The equilibrium constant K is actually the free formation of Cr 23 C 6 , increasing the Cr content can reduce r c and r c r , so it can be known from the formula ( 2 ) that increasing the Cr content in the steel can make the intergranular chromium-deficient The increase of the equilibrium Cr content in the zone is beneficial to weaken the tendency of intergranular corrosion of stainless steel.
Ni can increase the activity coefficient r c of C , so it can be seen from the formula ( 2 that, when other factors remain unchanged, X cr will become smaller with the increase of Ni content; on the other hand, increasing the Ni content will reduce the Solubility in austenite, therefore, increasing Ni content will cause C to form Cr 2 3 C 6 precipitation at the grain boundary and lead to severe chromium depletion in the intergranular, which is not conducive to the intergranular corrosion resistance of stainless steel.
In order to improve the mechanical properties of low-carbon stainless steel [ w ( C = £0.03% ]), adding a small amount of N during smelting can increase the yield strength to 0.08%w ( C level. N is similar to C and can form MC type The compound TiN weakens the carbon fixation effect of Ti and is detrimental to the intergranular corrosion resistance.
Due to the grain boundary adsorption phenomenon , the impurity elements P and S are easy to segregate at the grain boundary, which increases the intergranular corrosion tendency of stainless steel.
- Sensitization time
Taiyuan Iron and Steel Stainless Steel adopts hot metal pretreatment + converter + VOD (vacuum oxygen blowing decarburization process) when smelting TP321 steel grade. The C content of the finished TP321 stainless steel seamless pipe can be controlled at an ultra-low level ; Intergranular corrosion can allow the content of C ; P , S content is controlled in a lower range (Table 1. According to the temperature - time - sensitization curve and the intergranular corrosion empirical equation, intergranular corrosion begins to occur The sensitization time tc when corrosion ( M0.025 cm/a) is :
tc =63.5w(Cr -50.5w(Ni -6 820w(C -
Cr 23 C 6 on the grain boundary , and reduce the intergranular corrosion resistance of the steel.
In summary, it can be considered that high temperature, uneven cooling, and surface carburization during heat treatment are the factors that affect the intergranular corrosion resistance of TP321 stainless steel seamless steel pipe.
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