Ostenitik paslanmaz çelikte Nb ve Mo elementinin kaynak bölgesindeki etkisi

Abstract

Ostenitik paslanmaz çelik reaktör giriş hattının kaynaklarında ana malzeme ile uyuşabilecek elementler veya bu elementlerin yerine geçebilecek elementlere sahip kaynak dolgu malzemesinin kullanılması esastır. Yüksek basınç ve sıcaklıkta çalışan içerisinde hidrojence zengin olan gaz ve mayi geçen 321 kalite paslanmaz çelikte farklı dolgu malzemesinin kullanılmasıyla meydana gelecek sorunların çözümüne çare bulmak için bu çalışma yapılmıştır. Bu malzemeler özel alaşımlı malzeme olması ve kaynak edilebilmeleri hem zaman hemde ekonomik yönden büyük bir yük getirdiğinden dolayı üzerinden önemle durulmalı. İşe bilimsel olarak, mekanik, korozyon ve radyografik olarak tüm testler yapılmak suretiyle başladı. Sonuç olarak çok kısa zamanda olmasa bile uygun zamanlarda bu tür malzemelerinin orjinal dizayn malzemesiyle değiştirilmesinin gerekliliği tespit edilmiştir.

In petroleum refineries there exists high technology production units, important equipment's and pipe lines. In this units, almost each equipment, reactors, heat exchangers and furnaces have different grade materials. Especially furnace outlet, reactor inlet lines were made of Austenitic stainless steel. Among these lines, a search has been done on 321 grade stainless steel line, for minimising the dangerous operation conditions due to use of wrong electrodes at that line, in an other words for preventing the risk of fire and production lost due to the cracks on the line. Since it will take too much time to cut the line and re-weld with original electrode, this search has been done to determine scientifically how long can we go safely with existing faulty welds. As a result of performed tests, using of Mo electrodes instead of Nb electrodes the suitability of weld diffusion to material was searched. Finally it was concluded that on austenitic stainless steel there will be no crack occurring for short period but for long period the original materiel should be used. It was observed, with the work performed by electronmicroscope, that there exists high stress at the regions where the granular formation is sharp in the photos 367 and 368 for that reason this section should be re-newed. The necessary precautions will be cleared out, by scientific examination, that the risk due to use of wrong electrodes which was the main problem fared by usage of 12" Sch 160 321 grade steel (33.32 mm wall thickness) at the inlet line of this unit's reactor. By this work, an approximated time for changing the line will be given, due to the structural change that was recognised at closer time of start up of the unit since the repairment of line by cutting and re-welding will take too much time. This work has been performed, to remove the dangerous conditions due to usage of wrong electrode, in another words, to remove the bad conditions such as gas, liquid and being loss due to the of welds with wrong electrodes. At the 12" Sch 160 321 stainless steel (33.32 mm wall thickness) lines of the high technology unit, "Hydrocracker Unit", the pipes were welded with wrong electrode. Since the start-up time is close, changing of welds postponed to future times, or shutdowns. This work performed to observe the risky condition at environment due to wrong electrode. In these lines, % 94 purity of Hydrogen is circulated at rate of 172,000 Nm3/h, The lines also includes dense H2S which is very corrosive. For this reason, the test samples were examined by mechanical tests, corrosion tests and micro structure examination. As a result of analysis of 321 lines, at some welds, Mo was found with high ratio than limits, and at some other welds (Ti+Nb) stabiliser element found at inadequate ratio and some 304 quality welds without any stabiliser elements. Ni+Cr with standard ratio, was found at almost all the welds checked with surface machining. Because of this, Ni+Cr ratios were not considered in our examination. The high ratio of (Mo) element results in increase of hardness and hence decreases the tensile strength by 10 %. (Ti+Nb) element is used to prevent the acid corrosion between grains. This will be added to the material at a ratio of 4* carbon ratio or 5* carbon ratio according to some standards. The classical % 18 Cr., 8 % Ni low carbon sells are formed only in Austenitic phase. The existence of carbon dilutes the complex carburs, together with the gamma phase. Austenitic region develops with the condensitiy of Ni. This situation is obvious, especially at 1000 °C and shows the ability of melting of carburs increases with the increase of Ni ratio (Eg. at 1200 °C the limit is for 18/8 steel is % 0,4 and for 18/12 steel is & 0.60) Although the Ni in the ratio for % 8 is enough for the structure approximately under 1150 °C to be completely Austenitic and to stay stable up to the environment temperature, because of the reasons below for the steel with % 18 Cr, the Ni ratio is % 9 or more. VI For % 8 Ni steel by plastic forming at surround temperature, the Austenitic structure can be transformed partially martensite. An increase of Ni, that adjust the stability of austenite, prevents this transformation. This transformation result bad conditions at manufacturing. Due to the high mechanical strength of austenite at high temperatures, the hot forming of these steels (rolling, forming) should be performed at high temperatures. The existence of ferrite amount in austenite prevents these transformations takes place under good conditions. Since (Peritectic) reactions are slow, the structures corresponding to the (fiziko-simik) equilibrium is obtained rarely by homogenities can be formed by forming of unstable austenite with stable delta ferrite of cooling. An increase of Ni, decreases these possibilities. The weld of steels are not sensitive to the crocks of martensitic steels due to structures. Austenitic welds, at some conditions, can be sensitive to cracks types of following, Crocks results at high temperatures (above 1200 °C ) while cooling. Since the metals structure can not be hrown at this temperature, the examination of this type of cracks is very difficult. Cracks formed between dendrits, that is they follow the crystal interface of cooling structure. Generally it is observed that the completely Austenitic weld are the most sensitive to crack after the complete cooling. If all the other conditions are same, as the ferritic capability of the molten metal increases, its sensitivity to crack at the hing temperature decreases till it is disappeared. According to an obvious identification, there is such a ferritic ratio at the molten metal after cooling that the metal above this ratio is not sensitive to crack anymore. This situation is observed experimentally. Intergranular corrosion is a localized attack along the grain boundaries of a metal or alloy. Corrosion can proceed to the point where whole grains of metal fall away and the metal loses its strength and metallic characteristics. Intergranular corrosion is usually caused by an improper heat treatment or heat from welding that causes the precipitation of certain alloy components at the grain boundary. This precipitation causes a depletion of corrosion-resisting elements in Vll the area surrounding the grain boundary, and this art; a becomes anodic to the remainder of the grain. The austenitic stainless steels comprise the class of materials in which this form of attack is most common. The precipitate is a chromium carbide that appears at the grain boundaries during heating between 800 and 1400°F. The depleted component is chromium and attack occurs in the chromium depleted areas. There are three methods of combating intergranular corrosion in cases where susceptible materials must be heated in the sensitizing range. The first method is to reheat the metal to a temperature high enough to redissolve the precipitated phase and then cool quickly enough to maintain this phase in solution. The second method, called stabilization, is to add certain elements such as columbium, tantalum, and titanium in order to make use of their ability to combine more readily than chromium with carbon. In this way chromium is not depleted and the metal retains its corrosion resistance. The third is to restriet the amount of one of the constituetnts of the prceipitate-usually carbon-and thereby reduce the extent of the precipitation and resulting alloy depletion. Although it has been thougt that straining a metal would make it less noble and therefore subject to more rapid attack, it usually does not. Severely cold worked Nickel corrodes at approximately the same rate as when fully annealed. Some alloy sytems do show higher general corrosion rates after straining, but it appears that this increase is caused not by any significant shift in metal potential due to stress but by a metallurgical change in structure. It has been observed that on a metal sample with a sheared edge, which is a highly stressed area, the edge is more susceptible to pitting than the rest of the sample. It is just as likely, however, that pitting, in such cases, originates within the multitude of small crevices which exist as a result of shearing and is not directly related to the increased stresses. There are very important instances of stress and corrosion operating simultaneously to cause, not increased general attack, but fracture. Two such failure mechanisms are corrosion fatigue and stress-corrosion cracking. vm Metas that fail as a result of being alternately or cyclicly stressed are said to fatigue. Failure is by transgranular cracking (see Figure 12) and is usually only a single crack. (High temperature fatigue is intergranular since, above the equicohesive temperature, grain boundaries are weaker than the grains. A few metals such as lead and tin have low equicohesive temperatures and fail intergranularly even at room temperature.) Endurance limit and fatigue strength are measures of a metal's ability to withstand cyclic stressing in air. When the metal is cyclicly stressed in corrcisive environments, the joint action of corrosion and fatigue greatly intensifies the damage. Cracking is again transgranular, but there are usually a number of craeks and they quite often begin at the base of a corrosion pit. Fatigue data determined in air are useless as a design criterion for a part to be placed in service in a severe corrosive environment. Unfortunately, corrosion fatigue data for environments other than water or sea water are almost totally lacking. The most important consideration in selecting a metal for resistance to corrosion fatigue is the resistance of the metal to the corrosive environment. Metal strength is usually secondary.