The stainless steel heat exchanger for some plants. In the use of more than a year after the nozzle producing perfoliate crack. Affected the daily production of the factory. Making a serious security hidden trouble. Crack from the beginning of the tube bulge it, to another tube Kong Yanshen gradually, at the junction of nozzle have rust package. After stripped wrappage visible with the naked eye observation tube more than exist around the crack, the crack length more than 600 im. To find the causes of cracks and put forward the corresponding improvement measures. The chemical composition, fracture morphology, the tube material medium environment, corrosion products, such as microstructure was analyzed, and the comprehensive analysis of the cause of the crack.

A, test analysis

1, pipe material composition analysis

For nozzle sample chemical composition analysis, and compared with standard GB4237 the stainless steel hot rolled steel plate) in 1 crl8ni bases I composition control, the analysis results show that the actual material compared with national standard material, content of Ti, C content is slightly on the high side, tube material close to 1 crl8ni9 actually, rather than 1 cd8ni Si.

2, medium environment analysis

Heat exchanger used in industrial water steam from the factory of cycle. Heat exchanger is used. Field of heat exchanger is the medium of heating steam condensate and oil sampling inspection, results showed that the moisture analysis. Steam condensation water containing more Cr. This may be related to mix the Cl - in the process of industrial water cycle.

3, all phase analysis

Width different macro check nozzle, found the perfoliate crack, crack width is most the breadth to amount to 1.5 cafe. Making (not damage fracture morphology, fracture specimen tube mouth most metals has lost its luster, multiple steps, uneven fracture, fracture performance characteristics of brittle fracture. Microscopical observation specimen polished surface. The intergranular cracks found more, from coarse to fine gradually expand outside-in: at one end of the coarser non-metallic inclusions exist obvious; Also found a few areas along the crystal distribution of non-metallic inclusions. Modified: / trunk/SeaLandPlatform/GeneratedFiles/Debug/moc_SeaLandGLWidget CPPubstrate of austenite. More carbide particles on its distribution, and also present the distribution of the carbide along the crystal areolar tissue. The micro cracks along the grain boundary extension of carbide precipitation, crack characteristics conform to intergranular corrosion (mesh) and the characteristics of the stress corrosion (along the crystal).

4, analysis of corrosion products

Corrosion product of nozzle hole gap between sampling. With energy dispersive X-ray spectroscopy (EDS) analysis, the results showed that the corrosion products of the main ingredients for Fe, Cr, Cr local enrichment phenomenon.

Second, the corrosion mechanism is analyzed

1, the material influence

The corrosion resistance of stainless steel is mainly due to the surface with a layer of passivation film, improves the electrode potential, can prevent the corrosion reaction. And the bad, the pipe material of titanium and carbon content on the high side,

Grain boundary poor chromium phenomenon. Production state a passivity micro battery life. Titanium content in general 1 cd8ni Qu 10 times for the carbon content of 5 a. In this way can play a stabilizing role. But the material titanium content is too low, and carbon content on the high side. If the material used in manufacturing or process integrated by allergy transition temperature zone. Unstable carbon is easy to consume large amounts of chromium in the grain boundary formation of chromium carbide precipitation, internal carbon to the grain boundary diffusion speed is faster than chrome. Grain boundary chromium not complement the grain boundary in poor chrome area (i.e., sensitizing) form the passivation film, the passive state grain boundary is damaged, the electrode potential drop (anode, active state), and the grain itself remained passive state, higher potential (cathode), grain and grain boundary living state in a passive state micro battery, and has a big a small area ratio of anode, cathode under the influence of the coupling acceleration effect. Lead to intergranular corrosion materials, type intergranular stress corrosion resistance at the same time also fell.

2, medium environment and stress factors

Stainless steel stress corrosion phenomenon is common. Stress corrosion of stainless steel should have three basic conditions: the sensitivity of the alloy (material), the static tensile stress (mechanical factors) and the characteristics of the medium (environmental factors).

A pair of sensitive alloy lCd8Ni new moon. In chloride, hydroxyl and even more in the medium of sulfuric acid. All can cause stress corrosion. Static tensile stress a if there is no static tensile stress, even with the sensitivity of the alloy with a specific medium to cooperate. Stress corrosion is not going to happen. The stress corrosion of stainless steel is a major role in macroscopic internal stress. The residual tensile stress, rather than the micro internal stress. Heat exchange tube and tube plate, after expanding existing bilge meet force (residual tensile stress).

Characteristics of the medium especially some impurity in the solution. The existence of the so-called characteristic ions is the most dangerous. In 200 ℃ water only contains 2 PPM Cr, can make the austenitic stainless steel produces stress corrosion. And most of the origin for pit corrosion, crevice corrosion.

Heat exchanger steam report showed that the content of Cr is enough to make stainless steel to produce when in 200 a 300 ℃ intergranular stress corrosion. EDS analysis results also show that the crack surface has obvious local enrichment of Cr.

3, the formation of occluded cell

By expanding gap within the solution in a state of stagnation. Oxygen can only transfer within the seam in the form of diffusion. Make the oxygen consumption is difficult to get inside the seam. So that the gap in the cathode reaction aborted. However, cracks in the anode reaction to continue and to form a high concentration of metal ions solution cavity. To remain neutral solution, negatively charged anion (Cr) migration of people within the cavity, the generated metal chloride and hydrolysis reaction, acid in the cavity. Cause passivation membrane rupture, empty cavity electrode potential decreased, formed as the anode, the outer surface of the cathode. Especially Cr with H + generated hydrochloric acid. Its corrosion resistance stronger. Accelerated the corrosion rate of empty cavity. Because of a shortage of qin content in the material. Also easy cause activation of passivity. As the corrosion and corrosion products deposit. Formed in the crack occluded cell corrosion. And under the joint action of stress and corrosion. Extended to the further development of the crack. Heat exchanger

4, metallurgy and heat treatment

Microstructure shows for austenite plus a lot of not dissolve carbide. Exist along the crystal distribution of the carbide network at the same time. Indicated that the materials without solid solution treatment, or solid solution treatment temperature is not high, the time is shorter, carbide not fully dissolve the gills, alloying degree is poor. And, low heat treatment temperature, chromium diffusion at a slower pace, chromium carbide precipitation is accelerated, more accelerated the formation of the poor chrome area, reduce the electrode potential. At the same time. Along the crystal distribution of chromium carbide exacerbate grain boundary caused by poor chromium stress corrosion along the grain boundary.

The existence of the intergranular non-metallic inclusions indicate that the material grain is very rough. Coarse grain tendency of intergranular corrosion. The reason is coarse grain per unit volume of grain boundary area is small, and under the condition of given sensitization of carbide precipitation is certain. The coarse grain of grain boundary area density is larger than of fine-grain carbides. The coarse grain size, on the other hand, has prompted acceleration of M23C6 precipitation effect, at the same time the intergranular distribution of non-metallic inclusions fragmented the continuity of matrix, reduces the strength of the material. The tip of the non-metallic inclusions are caused by stress concentration. Heat exchanger

Three, endnotes

Nozzle material analysis and microstructure analysis show that the material is not required. Result in matrix and grain boundary type between the resistance to intergranular corrosion and stress corrosion performance degradation. Medium Cr in grain boundary sensitization and residual stress under the joint action of intergranular type caused by stress corrosion, and Cr viscous flow in the tube to tubesheet gap. Under the effect of residual stress, acid autocatalytic cause crevice corrosion, and the intergranular non-metallic inclusion lead to stress concentration, organization, coarse grains, also contributed to the emergence and development of corrosion. To prevent crack, can be improved from the following four aspects:

1, material control

Strictly control the quality of material, by adding a stabilizing element or lower carbon content. Control of grain boundary adsorption and inhibition of grain boundary precipitation: heat treatment temperature, time and temperature change speed must meet the requirements, to reduce the precipitation of chromium carbide, control of grain size and fine grains, reduce the probability of corrosion.

2, improved structure

Try to avoid formation of block space. The design of tube plate and pipe gap can consider to ask seal processing. Recommended sealing welding, and to eliminate stress after welding.

3, working condition of medium control

Strictly control the impurity in the medium. Especially C1. The content, but through technological means to control it under the concentration of caused by stress corrosion.

4, select material to consider

Because lCrl8Ni bases C content is higher in the I. Will increase the sensitivity of stress corrosion of stainless steel, as well as in the equipment manufacturing process after hot working or welding thermal cycle. The stabilization effect of Ti can be decline or even disappear. Easy to cause grain boundary sensitization. Degrade the performance of the intergranular corrosion resistance, therefore in the pressure vessel industry. Now do not recommend using the steel grade. Can choose lower carbon content of Cr - Ni austenitic stainless steel. Such as 0 cd8nil0ti steel, OCrl8 NiUNb or ultra-low carbon steel stainless steel 00 cd9nil0 steel, etc., in order to reduce the harm of the grain boundary sensitization. Also can consider to use duplex stainless steel, stainless steel to reduce the stress corrosion cracking sensitivity of chlorine ion.