Acier inoxydable 304 VS acier inoxydable 321

/dans /par

Les grades 304 et 321 appartiennent tous deux à la série d’acier inoxydable Austenitic 300. Ils présentent des caractéristiques similaires en matière de résistance à la corrosion, de résistance à la rupture, de dureté et de soudabilité, mais le grade 321 est principalement utilisé dans les conditions de résistance à la chaleur de 500 à 600 °C. L’acier inoxydable 321H est la version à faible teneur en carbone de l’acier 321, est l’acier résistant à la chaleur le plus utilisé, dont le contenu en carbone est légèrement supérieur à celui des grades 321. Acier 304 C’est un matériau de remplacement de l’acier inoxydable 321 lorsque l’on recherche une résistance à la corrosion intergranulaire plutôt que de la résistance à haute température.

D'une certaine manière, l'acier inoxydable de qualité 321 est une nouvelle version basée sur Grade 304 En ajoutant du Ti pour améliorer la résistance à la corrosion des zones de transition des grains et la résistance à hautes températures. En tant qu’élément stabilisateur, le Ti contrôle la formation de carbure de chrome, ce qui rend le 321 très résistant à hautes températures, bien plus que le 304 et le 316L. Une teneur plus élevée en nickel confère au 321 une bonne résistance à l’abrasion à différentes concentrations et températures d’acides organiques, notamment dans des milieux oxydants. Acier inoxydable 321 Il présente une meilleure propriété de rupture à la pression et une résistance à la fatigue mécaniques comparées à l'acier inoxydable 304. Je vous montre exactement la différence entre eux avec les deux tableaux ci-dessous.

 

Composition chimique des 304, 321 et 321H

Notes C Oui Mn Cr Ni S P N Ti
304 0.08 1.0 2.0 18.0～20.0 8.0～10.5 0.03 0.045 / /
321 0.08 1.0 2.0 17.0-19.0 9.0-12.0 0.03 0.045 0.1 5C-0,70
321H 0.04-0.1 1.0 2.0 17.0-19.0 9.0-12.0 0.03 0.045 0.1 0.16-0.7

 

Propriétés mécaniques des aciers 304 et 321

Notes Résistance à la traction, MPa Résistance à la traction, MPa Extension, % Dureté, HB
304 ≥520 205-210 ≥40≥40 HB187
321 ≥520 ≥205   HB187

 

Comme on peut le constater dans le tableau ci-dessus, l'acier inoxydable 321 contient du titane et davantage de nickel (Ni) que l'acier inoxydable 304, selon la norme ASTM A182. Le contenu de Ti ne doit pas être inférieur à 5 fois la teneur en carbone (C), mais ne doit pas dépasser 0,7%. Le titane peut prévenir la sensibilisation de l'acier inoxydable et améliorer la durée de service à haute température, c'est-à-dire : grade 321 Il convient mieux à la fabrication de récipients à base acide résistants à l’usure, d’équipements résistants à l’usure et de tuyaux de transport ou d’autres pièces que l’acier inoxydable 304 dans un environnement à haute température.

Les aciers inoxydables 304 et 321 peuvent être utilisés dans les domaines de la chimie, du pétrole et du gaz, ainsi que dans l’industrie automobile. L’acier inoxydable 304 est un acier inoxydable polyvalent et possède les applications les plus étendues dans la famille des aciers inoxydables, telles que les ustensiles de cuisine, les armoires, les chaudières, les pièces automobiles, les appareils médicaux, les matériaux de construction, les produits chimiques, l’industrie alimentaire, l’agriculture, le transport maritime, le transport de pétrole et ainsi de suite. L’acier inoxydable 321 est utilisé dans les domaines de la chimie, du charbon et du pétrole, où il est nécessaire de résister à la corrosion des limites des grains ainsi qu’à des propriétés à haute température telles que les tubes d’échappement des moteurs, les tubes d’échappement des moteurs, les coffrets des chaudières, les échangeurs de chaleur, les composants des fours, les silencieux des moteurs diesel, les cuves de pression des fours, les réservoirs de transport de produits chimiques, les joints d’étanchéité, les tubes des fours, etc.

Pourquoi le tube en acier inoxydable nécessite un recuit de solution ?

/dans /par

Solution annealing is also referred as carbide solution annealing, is a process that heats the work part to 1010℃ or above to remove of carbide precipitation (Carbon from the stainless steel solid solution), and then it is rapid cooling, usually, water quenching and the carbide returned to the stainless steel solid solution. Solution annealing treatment can be applied to alloy steel and stainless steel. For 304 stainless steel castings, solution treatment can produce uniform microstructure without carbide impurities. Generally, the stainless steel tube is heated to about 950 ~ 1150℃ for a long time to make the carbide and various alloying elements fully and evenly dissolved in Austenite, and then quickly quenched water cooling to obtain pure Austenite structure due to carbon and other alloying elements to late precipitation. There comes with the question, why the stainless steel pipe needs solution annealing? Firstly you should know the function of the solution annealing process.

Uniform metallographic structure

This is especially important for raw materials. Inconsistencies in rolling temperature and cooling rate of hot rolled steel tubes cause the same consequences in the structure. When atomic activity increases at high temperatures, σ dissolves and chemical composition tend to be uniform, then a uniform single-phase structure is obtained after rapid cooling.

 

Elimination of work hardening

The solid solution treatment restores the twisted lattice and recrystallizes the broken grain. The internal stress and tensile strength of the steel tube reduce while the elongation rate increases to facilitate the continuous cold working.

 

Increased corrosion resistance

The corrosion resistance of stainless steel decreases with the precipitation of carbide, and the corrosion resistance of steel tube returns to the best after solid solution treatment. Temperature, holding time and cooling rate are the most important factors in solution treatment for stainless steel.

The solid solution temperature depends on the chemical composition. Generally speaking, the solid solution temperature should be correspondingly increased for the grade with more alloy elements and high content, especially for the steel with a high content of manganese, molybdenum, nickel and silicon. Only by raising the solid solution temperature and making it fully dissolved can the softening effect be achieved.

However, there are some exceptions, such as 316Ti. When the solid solution temperature is high, the carbide of the stabilized elements is fully dissolved in the Austenite, which will precipitate out at the grain boundary in the form of Cr23C6 and cause intergranular corrosion in the subsequent cooling. The lower solid solution temperature is recommended to prevent carbide (TiC and Nbc) of stabilizing elements from decomposition and solid solution.

 

Why does stainless steel corrode?

/dans /par

As we all know, stainless steel has the ability to resist atmospheric oxidation, that is, will not rust, but also corrode in the medium like acid, alkali and salt, that is, corrosion resistance. However, the corrosion resistance of stainless steel is conditional, that is, stainless steel in a certain medium is corrosion-resistant, but in another medium may be destroyed. Correspondingly, no one of stainless steel is resistant to corrosion in all environments.

Stainless steel can provide excellent corrosion resistance in various industries, strictly speaking, they show excellent corrosion resistance in most media, but it is exceptional in some media due to low chemical stability and corrosion but. Therefore, stainless steel can not be corrosion-resistant to all media except mechanical failure. The corrosion of stainless steel is mainly manifested as a serious form of corrosion of stainless steel is local corrosion (i.e., stress corrosion cracking, pitting, intergranular corrosion, corrosion fatigue and crevice corrosion). This local corrosion causes almost half of the failure. To understand why stainless steel corrodes, we must first understand the type of corrosion of stainless steel.

 

Stress Corrosion Cracking (SCC)

Stress corrosion cracking (SCC) is the failure of stainless steel subjected to stress in a corrosive environment due to the expansion of strong grain. SCC has a brittle fracture morphology and can occur in materials with high toughness in the presence of tensile stress (whether residual stress or applied stress or both) and corrosive media. In the micro term, crack through the grain called transgranular crack, and the cracks along the grain boundary expansion graph called the intergranular crack, when the SCC extended to one depth (load stress on the section of materials to achieve its fracture stress) in the air, stainless steel as normal crack (in ductile material, usually through microscopic defect aggregation) and disconnect.

Therefore, the section of a part that has failed due to stress corrosion cracking will contain areas characterized by stress corrosion cracking and “dimples” areas associated with the polymerization that has been slightly defective.

 

Pitting Corrosion

Pitting corrosion refers to the most non-corrosion or scattered slight local corrosion on the surface of metal materials. The size of the common pitting point is less than 1.00mm, and the depth is often greater than the surface aperture, which may be a shallow pitting pit or perforation.

 

Intergranular Corrosion

Intergranular corrosion: A disordered dislocation of grains at the boundary between different grains and, therefore, a favorable zone for segregation of solute elements or precipitation of metallic compounds such as carbides and δ phases in steels. Therefore, in some corrosive media, it is common that the grain boundaries may be corroded first, and most metals and alloys may exhibit intergranular corrosion in certain corrosive media.

 

Crevice Corrosion

Crevice corrosion refers to the occurrence of speckled corrosion in the cracks of stainless steel parts, which is a kind of local corrosion. It may occur in the cracks of solution stagnation or in the shielding surface. Such gaps may form at metal-to-metal or metal-to-nonmetal junctions, for example, at rivets, bolts, gaskets, valve seats, and loose surface deposits.

 

General Corrosion

Uniform corrosion on the surface of stainless steel. Stainless steels may exhibit general corrosion in strong acids and bases. When general corrosion occurs, the stainless steels gradually thin and even fail, which is not much of a concern because such corrosion can usually be predicted by a simple immersion test. It can be said that stainless steel refers to the corrosion resistance of steel in the atmosphere and weak corrosion medium, corrosion rate is less than 0.01mm/ year, that is “completely corrosion resistance”; Stainless steels with corrosion rates less than 0.1mm/ year are considered “corrosion-resistant”.