In this article, the related information about superheater tubings manufactured from chromium steel steels is presented. Three subdivisions are included: specific composings of two typical chromium steel steels, 304 and 316 ; the processing paths of two sorts of tubings, seamless tubings and welded tubings ; three microstructural characteristics, i.e. grain boundaries, sigma stage and retained ferrite, and the affect ensuing from them.The Applications of Stainless Steels in Superheater Tubes
A superheater is one of the most of import accoutrements in a modern boiler which is widely used in engines and power workss. In a boiler, the wet constitutes the basic restriction in the boiler design, and the major map of the superheater is to change over the moisture and concentrated steam into superheated steam which is dry and possesses higher thermic energy [ 1-2 ] . In this manner, the overall efficiency of transition of heat to work is increased and the likeliness of steam condensation and eroding is eliminated [ 3 ] . Normally, a superheater comprises a group of parallel tubings, the surfaces of which are exposed to the heated gases from the furnace to take the hint of wet and to superheat the wet steam. One typically perpendicular superheater is shown in Fig.1.
Fig.1 A perpendicular superheater [ 4 ]
Harmonizing to the description above, it can be seen that the superheater tubings are subjected to elevated temperature ( normally above 550oC ) and to high force per unit area ( above 2000 pounds per square inch ) [ 5 ] . Therefore, the primary demands for the technology alloys that may be used include high weirdo resistant at elevated temperature, outstanding high temperature anti-corrosion and anti-oxidation belongingss. Furthermore, good processing features are besides required because of its complicated form.
Harmonizing to American Society for Testing Material ( ASTM ) specifications, a figure of classs of austenitic chromium steel steels are intended for superheater tubings [ 5, 6 ] . Typical illustrations include TP304, TP316 and TP321. In add-on, some alterations of these classs, such as TP304H, TP316H and TP321H, are besides widely applied in superheater tubing fabrication. In the undermentioned subdivisions, two types of austenitic chromium steel steels, 304 and 316, will be discussed as illustrations in item, including their composings, treating paths and microstructural characteristics.
Stainless steel steels are iron alloys incorporating Cr more than 12 % by mass. For austenitic chromium steel steels, Ni is another major debasing component in add-on to Cr. The nominal composings for TP304 and TP316 are listed in Table 1 and Table 2, severally.
Table 1 Nominal composing of TP304 chromium steel steel [ 7 ]
Table 2 Nominal composing of TP316 chromium steel steel [ 7 ]
In add-on to the elements listed in the tabular arraies above, some other little sum of elements, such as phosphoric ( 0.045 % ) and sulfur ( 0.03 % ) , are besides contained in these two steels.
In comparing to normal steels, it is apparent that a figure of elements are added into unstained steels in order to better relevant belongingss, including mechanical strength and corrosion opposition.
Chromium as the cardinal alloying component in unstained steels normally can organize a protective bed above the steel substrate. This bed is normally considered as chromia ( Cr2O3 ) . Harmonizing to the old research, the potency of O is decreased by the formation of chromia, therefore the oxidization of metals [ 8 ] . The formation of the surface movie consequences in the depletion of Cr at the substrate/scale interface [ 9 ] . In order to keep this bed, sufficient Cr is required in the chromium steel steels ; therefore the content of Cr in these two metals reaches up to 18 % about. In general, the corrosion opposition is improved with the content of Cr. However, it should be noted that in most instances the protective beds consist of Fe and Cr oxides instead than pure Cr2O3.
Nickel is the 2nd largest sum of component in these two chromium steel steels mentioned above. The add-on of Ni has a marked good consequence on oxidization opposition in impersonal oxidising media. By and large talking, nickel influences the adhesion and mechanical belongingss of the graduated table [ 10 ] and reduces the diffusion rate of the cation in the chromia bed [ 11 ] . In add-on, Ni is the most of import austenite stabilizer which extends the austenite stableness scope hence retaining austenite at room temperature [ 12 ] .
Silicon Acts of the Apostless as an effectual heat-resistance component which is normally present in unstained steels as a residuary dross alternatively of a calculated additive. Manganese is normally regarded as an austenite stabilizer, although it was reported that its presence had an inauspicious influence on oxidization opposition [ 13 ] .
Comparing the two tabular arraies above, the major difference between two types of steels is that 2-3 % Mo is added to TP316 chromium steel steels. The primary map of Mo is to heighten the roughness and cranny corrosion opposition of chromium steel steels.
Harmonizing to the standard specifications of ASTM, two classs of austenitic steel superheater tubings are manufactured: seamless tubings and welded tubings [ 5, 6 ] . Different processing paths are required for two superheater tubings.
The major production stairss for fabricating seamless tubings are illustrated in Fig.2. Before the production procedure, the chromium steel steels are produced into notes, which are converted from blooms and cut by winging shears into certain lengths.
Fig.2 Basic stairss in fabricating seamless tubings [ 14 ]
Harmonizing to the figure above, it is indicated that the seamless tubing fabrication is began by heating the notes. The cylindrical notes are heated to 1200oC about in a rotatory furnace. Then the white-hot note is positioned in the rolled line and drawn by axial rotations over a piercing rod. The perforation procedure is illustrated in Fig.3.
Fig.3 Schematic of perforation [ 15 ]
After the piercing phase, the tubings are elongated and hot-rolled or cold-rolled. Finally, the tubings are straightened and cut into required size [ 14 ] .
It should be noted that in many instances, austenitic chromium steel steels for illustration TP304 and TP316, contain some ferrite forming elements, such as Si and Mo ( in fact, Cr is besides a strong ferrite former ) . The presence of these elements may take to the formation of little sum of ferrite during turn overing or hammering procedure. Normally, the presence of ferrite makes the corresponding procedure hard, because the ferrite atoms may transform to sigma under certain conditions, for illustration the operating temperature is comparatively low [ 12 ] . For this ground, the content of Ni is raised to 10 % to 14 % for chromium steel steels incorporating Mo, one illustration of which is TP316 [ 12 ] .
The primary stairss in fabrication welded unstained steel tubings are shown in Fig.4.
Fig.4 Basic stairss in fabrication welded tubings [ 14 ]
Harmonizing to Fig.4, it can be seen that welded tubings are manufactured by turn overing metal strips, which are besides called skelps. The chromium steel steel home bases are positioned on an unwinding machine ; so the home bases are passed through rollers to curve them together. After that, the tubings are automatically welded by assorted welding procedures. Based on ASTM Standard specification A249, the tubings should be welded with no additives or filler metals [ 6 ] . In general, post-heat-treatment is applied to welded tubings for emphasis alleviation, because stress-corrosion snap is one of the of import signifiers for corrosion harm to superheater tubings. Similar with seamless tubings, the welded tubings are cut into required size and completed surface coating.
It should be pointed out that the most of import job for welded austenitic-stainless-steel tubings is sensitisation. The primary cause of sensitisation is chromium depletion in the part adjacent to the grain boundaries. When the austenitic chromium steel steels are welded, the temperature of the heat-affected zones is in the scope between 540 to 730oC about [ 7 ] . At this scope of temperatures, Cr carbides start to precipitate in the austenite constructions, where the grain boundary is the most likely part. Due to the different diffusion rates for Cr atoms and C atoms, Cr depleted parts are likely to organize near the grain boundary, which leads to the intergranular corrosion. In order to avoid sensitisation consequence, several methods are normally applied. The first manner is to take down the C content. In some instances, the extra-low-carbon classs of austenitic chromium steel steels are utilised, such as 304L and 316L, where L denotes low C content. The 2nd method is to increase the Cr content, in which manner the Cr depleted parts may be eliminated. Sensitization besides can be avoid by adding other little sum of debasing elements, including Ti and columbium. These carbide forming elements can unite with C much rapidly than Cr. The last attack to extinguish sensitisation is to reheat the chromium steel steel merchandises at certain temperatures at which the formed carbides can redissolve ; so it is cooled quickly [ 7 ] .
As the name implies, the major microstructure for austenitic chromium steel steels is austenite stage, in which several microstructural characteristics influence the end point belongingss, including grain boundaries, sigma stages and retained ferrite.
As described antecedently, the grain boundaries normally provide sites for intergranular corrosion which involves the Cr depletion at the parts adjacent to the boundaries, as shown in Fig.5. In malice of sensitisation mentioned above, grain boundaries besides play a function of heightening the oxidization opposition of chromium steel steels in some instances. This phenomenon can be explained that the rate of Cr diffusion to come up along boundary way is increased with the grain boundaries [ 16 ] .
Fig.5 Intergranular corrosion in TP304 chromium steel steel [ 17 ]
Sigma stage is an intermetallic compound which is difficult, brickle and non-magnetic, as shown in Fig.6. This stage is damaging to the relevant corrosion opposition, particularly opposing corrosion opposition, ductileness and stamina, so that applied scientists normally avoid unstained steels to be exposed at the temperature of 800oC about, which favours its formation [ 17 ] .
Fig.6 Sigma stage in austenitic chromium steel steels [ 7 ]
By and large talking, sigma stage signifiers easy in austenitic matrix, and obviously 18/8 austenitic chromium steel steels ( 18 % Cr, 8 % Ni ) are immune to sigma stage. However, the formation of sigma stage can be accelerated by the presence of ferrite and some other little sum of elements, such as Mo, Si and Ti [ 7 ] . Therefore, TP316 chromium steel steels which contain Mo may organize sigma stage at appropriate temperature. Similar with the procedure of desensitisation, sigma can be redissolved by heating up to 900oC or supra.
In add-on to grain boundaries and sigma stage, retained delta ferrite is besides normally present in austenite matrix, as shown in Fig.7, in which the retained ferrite is in a vermiculate morphology. As mentioned antecedently, the presence of ferrite may do trouble in hot working.
Fig.7 Optical micrograph of as-cast TP304 chromium steel steel microstructure [ 18 ]
The formation of ferrite commonly depends on the composing of the metals, but in some instances, it is besides produced in the dyer’s rocket bead. It has been realised that some sum of ferrite ( 4 to 8 % ) in dyer’s rocket bead is effectual to countervail the inauspicious consequence arisen from the grain boundary failing [ 7 ] . However, it should be noted that high ferrite content may take to high-temperature strength lessening and the formation of sigma stage.
In this article, it is presented that austenitic chromium steel steels are extensively utilized in superheater tubings. The specific composings of two typical chromium steel steels, TP304 and TP316 are described, followed by some elemental effects. Two chief processing paths for fabricating superheater tubings, i.e. seamless tubing processing and welded tubing processing, are described in item. Finally, the influences and characteristics of relevant microstructures including grain boundaries, sigma stage and retained ferrite are discussed..