The effect of different annealing temperatures and times on 316L bellow part of pipe compansator

Pipe compansators are used in plumbing systems of high buildings to eliminate expansion, contraction, vibration and noise problem occurred by the difference of temperature. Plumbing systems have carbon steel pipelines which transfer hot water, boiling water or hot steam fluids. Hot fluid goes up along pipeline and turn back as fluid having lower temperature. Temperature difference due to fluid effect causes expansion and contraction on steel pipelines. In the buildings, pipes of plumbing system operating 90/70°C have 3 mm expansion on each floor. Up to 7 floors (21 m), expansion on steel pipelines can be removed with the vent stacks and with the elbows in the column line. However, the expansion that occurs in the pipelines above 7 floor buildings can not be avoided by vent stacks and elbows, and expansion at this point becomes a serious problem. Compensation of expansion, contraction and vibration is realized by 316L stainlesss steel bellow part which is manufactured by cold working. 316L stainless steel sheet metal transforms to tube by means of tube machine. Sheet metal is first crushed and then bended on machine. The contacting surfaces after bending operation are welded automatically on machine. Thus, the stainless steel sheet metal tranforms to tube form. The bellow part is manufactured with double tube. Telescoped two tubes as close fit transform to bellow part by means of bellow machine which works pneumatically. Telescoped tubes are shaped to bellow part on machine as cold working. Due to cold working, bellow part has residual stresses in its structure and these residual stresses cause damage of pipe compansator during usage in plumbing system before its lifetime. Stress relieving heat treatment for austenitic stainless steel is preferred between 1050 °C and 1120 °C as slow or fast cooling. Austenitic stainless steels with percentage of 0.03-0.08% C have a risk of sensitisation between 550 C – 850 C. Sensitisation is associated with precipitation of chromium carbide such as Cr23C6 ya da Cr7C3 along grain boundaries during heat treatments. During carbide precipitation, carbon diffuses rapidly to the grain boundaries. Unlike carbon, chromium diffuses slowly, resulting chromium-depleted zone at the grain boundaries. Austenitic stainless steel then becomes deficient in chromium at the grain boundary region and no longer resists corrosion. So the steel is susceptible to intergranular corrosion. There are several ways to avoid sensitisation of austenitic stainless steels. The minimisation of carbon content or the addition of strong carbide formers such as titanium and niobium can prevent sensitisation. Since the austenitic stainless steel used for bellow part has low carbon content, no sensitisation risk exists and slow or fast cooling can be applied for cooling process. Stress relieving heat treatment as different annealing temperature and annealing time was applied on 316L stainless steel bellow part in order to eliminate the residual stresses so to increase fatigue life. 950°C , 1050°C and 1100°C were selected as annealing temperature and 8 min, 20 min and 40 min were selected as annealing time. Slow cooling was preferred for cooling process. Examining different annealing parameters' effects on mechanical properties of bellow part which is manufactured by cold working and determining their effects on product life were aimed with this study. Based on study, cold worked bellow part was taken as reference and cycle test, tension test and hardness test was applied to products annealled with 3 different annealing temperatures and 3 different annealing times. Corrosion test was applied on samples to observe corrosion behaviour after annealing processes. Additionally, metallograpfic inspection was performed on each sample in order to determine the effect of different annealing temperatures and times on microstructure. With the stress relieving heat treatment, it is predicted that the residual stresses in the material due to the cold working will be removed and therefore the fatigue life will increase, however this result could not be achieved during the fatigue test for every annealing temperatures and annealing times. For 40 min and 20 min annealing times, the number of cycles didn't show continuous increase or decrease with the rising annealing temperature. Even, some results were lower than non-annealed sample. However, for 8 min annealing time, the number of cycles increased by increasing of annealing temperatures and all number of cycles belong to 8 min annealing time were higher than non-annealed sample. Tension test results were plotted by determining the force values corresponding to different extents. Since the force values corresponding to the extension will also give information about the tensile strength of the material, the obtained force values are related to the tensile strength. Annealing process is expected to reduce the tensile strength since it removes residual stresses and make the structure more ductile. With the results of tension test, the forces obtained after anneling process were lower than non-annealed sample. However, no correlation between annealing temperature- time and force was detected as increasing temperature decreases tensile strength. Comparing to annealing times, 8 min annealing time had lower tensile strength values than 40 min and 20 min annealing times. With the hardness test, all hardness values after annealing process were lower than non-annealed sample's value. This result was expected since the hardness and tensile strength are related to each other. However just like tensile strength, no correlation between annealing temperature- time and force was detected for hardness values. Comparing to annealing times, 8 min annealing time had lower hardness values than 40 min and 20 min annealing times. Although the annealing process did not show any significant corrosion effect in the structure, it started the formation of corrosion marks in 20 min and 40 min annealing times regardless of the temperature. It has been observed that the material may be oxidized in the furnace by keeping it in the furnace for a long time. Since high temperatures will increase the system energy of the material and therefore increase the diffusion rate, oxidation tendency may occur in the material. Due to the long time exposure, this tendency can give rise to corrosion marks in the form of oxidation on the surface of the material. With the corrosion test, no corrosion was observed with 8 min annealing time at every annealing temperatures, however, corrosion marks were detected during 20 min and 40 min annealing times. As the annealing temperature increases, it is expected that the material grain size will grow. In the same way, the grain siz As the annealing temperature increases, it is expected that the material grain size will grow. In the same way, the grain size increases as the annealing time increases. The images obtained by metallographic examination showed the effect of heat treatment on the grain structure. As the annealing temperatures and times increase, the grain sizes increased in each sample. According to the results, it is indicated that annealing process increases fatigue life. The best result based on fatigue life was obtained with 8min – short time- annealing process. Annealing process removed residual stresses of bellow part which was manufactured by cold working. Due to heat treatment process, hardness and tensile strength of material decreased and fatigue life increased. For 8 min annealing time, fatigue life increased by increasing of annealing temperature. No corrosion detected on samples which were annealed with short time annealing. Contrary to 8 min annealing time, 20 min and 40 min annealing times did not improve fatigue life. Additionally, corrosion was observed on these annealing times. So it is noted that middle and long time annealing processes are not appropriate for 316L bellow part with regard to fatigue life. Therefore, in order to increase the fatigue life of the pipe compensators which are damaged earlier than expected, it is suggested to apply the stress relieving heat treatment to the 316L stainless steel bellows part in short time and about 1100 ° C. Additionally, in order to further increase the fatigue life, which is the main target, optimization can be carried out by performing experiments at annealing times above 1100 ° C as well as under 8 minutes. The influence of the fatigue life in the negative direction during middle and long time annealing processes is a controversial issue. Decreasing of the fatigue life with increasing annealing time may be related to the grain size. The grain size effect on the fatigue life should be examined in detail. Additionally, there was no clear relationship between tensile and hardness values in middle and long term annealing times. These mechanical properties should also be examined how they are affected by grain size growth.