Stainless steel welded pipe weld seam corrosion overview

 Stainless steel welded pipe weld seam corrosion overview

Corrosion of welded joints, usually occurring in stainless steel pipes, has three forms of corrosion.

① The welding seam is corroded into a spongy shape, which is the δ ferrite selective corrosion of austenitic stainless steel.

In order to improve welding performance, austenitic stainless steel usually requires the weld to contain 3% to 10% ferrite structure, but in some highly corrosive media will occur δ ferrite selective corrosion, that is, corrosion occurs only in the δ ferrite phase (or further decomposition into σ phase), the result is spongy.

② Heat affected zone corrosion. The reason for this corrosion is that the temperature here is just in the sensitized zone during the welding process, and there is sufficient time to precipitate the carbide, resulting in intergranular corrosion.

Intergranular corrosion is a kind of corrosion form where the corrosion is limited to and near the grain boundary and the grain itself is relatively small, which will cause the grain to fall off or reduce the mechanical strength of the material.

The mechanism of intergranular corrosion is "chromium deficiency theory". Stainless steel has high corrosion resistance due to chromium, and its chromium content must exceed 12%, otherwise its corrosion resistance is similar to ordinary carbon steel. In the sensitization temperature range of stainless steel (450 ~ 850℃), the susaturated solid solution carbon in austenite will be combined with chromium to form Cr23C6 and precipitate along the grain boundaries. Because the diffusion rate of chromium in austenite is slower than that of carbon, the lead required for the formation of Cr23C6 must be obtained from near the grain boundary, resulting in chromium deficiency near the grain boundary. If the chromium content is reduced to 12% (the limit of chromium content required for passivation) below, the chromium-poor region is in an activated state, as an anode, it forms a corrosion galvanic cell between the grain, the anode area of the chromium-poor region is small, and the cathode area of the grain is large, resulting in serious corrosion of the chromium-poor region near the grain boundary.

③ knife-edge corrosion at the fusion line generally occurs in stainless steel (347 and 321) stabilized with Nb and Ti. Knife-edge corrosion mostly occurs in oxidizing media.

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CANGZHOU GALAXY STEEL PIPE CO., LTD

 

China Reliable Steel Pipe and Pipe Fittings Manufacturer and Supplier

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Introduction of correlation between steel material properties and temperature

 There are two types of steel material coefficient related to temperature: one is steel material coefficient related to mechanical properties of steel materials; The other is the material coefficient of steel related to heat conduction. The former are E, G, v, a; Among the latter are C(specific heat capacity), ρ (density), k(heat conduction coefficient) and so on. These coefficients are not actually constant, but vary with temperature. However, when the temperature is not high, the average value is usually treated as a constant, but in the case of high temperature and large change, it must consider its change with the temperature.

Relationship between elastic coefficient and temperature of iron and steel materials

The elastic coefficient E and shear modulus G decrease with increasing temperature, and Poisson's ratio v changes little with temperature. E, G and temperature are measured by static method and dynamic method, the former is tested by loading in a high temperature furnace, the latter is measured by vibration method or ultrasonic pulse method. The vibration method is to make the specimen do elastic vibration in the high temperature furnace, and determine the elastic constant by measuring the frequency. The ultrasonic rule is to give the specimen ultrasonic waves, and measure E, G, v by measuring the propagation speed of the waves.

The relationship between heat coefficient and temperature

The thermal coefficient of metal materials is generally linear with the temperature, the linear expansion coefficient a generally increases with the increase of temperature, the thermal conductivity k decreases with the increase of temperature, and the specific heat capacity increases with the increase of temperature. Through the linear slope or curve of the relationship between the heat coefficient and temperature measured by the test, the change of the heat coefficient of the specific material with the temperature can be known.

Thermal fatigue of steel materials

When the ductile steel material increases with the temperature, even if the stress exceeds the yield point, it will not be immediately damaged, but even if the stress level is low, if there is a large temperature change repeatedly, it will eventually crack due to fatigue and lead to damage. This phenomenon is called thermal fatigue.

Assume that at the beginning of the test, the rod is fixed at the highest temperature and then cooled to produce tensile stress, with OAF being a stress variation line. Then, if the heat is reheated, the stress-strain line begins to move down parallel to OA, yielding at a stress lower than the cooling cycle tension, and finally reaching point E. If it is kept at the highest temperature for a period of time, the compressive stress decreases to the E' point due to stress relaxation. If it begins to cool again, it rises along E 'f ', reaching F' point at the lowest temperature. Since no pressure relaxation occurs at the lowest temperature. If the heating starts again, the line falls along F'E" to E" at the highest temperature. Here, due to stress relaxation, the stress decreases and moves to E"'. If cooling starts again, F" is reached at the lowest temperature along the curve E"'F".

If this cooling-heating cycle is repeated, the stress-strain diagram depicts a hysteresis curve each time, and the associated retroplastic strain is the cause of thermal fatigue. The maximum and minimum temperature, the average temperature, the holding time of the maximum temperature, the repetition rate, the elastoplastic property of the material are all factors that affect the thermal fatigue.

The strength of thermal fatigue refers to the relationship between the plastic strain εP of a cycle and the number of repetitions N to reach failure.

The above mentioned is only the unidirectional thermal stress fatigue of the material, and the thermal fatigue of the actual structure is multi-directional and is a special research field.

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CANGZHOU GALAXY STEEL PIPE CO., LTD

 

China Reliable Steel Pipe and Pipe Fittings Manufacturer and Supplier

 

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Various structures and inclusions in steel (Chapter Three)

 Standard for determination of non-metallic inclusions in steel

1. Scope

Microscopic evaluation method for nonmetallic inclusions in rolled or forged steels with compression ratio greater than or equal to 3 by standard spectrogram.

2 Sampling

The morphology of inclusions depends to a large extent on the degree of compression deformation of the steel, so it is only possible to compare the measured results on the sections prepared by sample billet after similar degree of deformation.

The polished surface area of the specimen for measuring inclusion content shall be approximately 200mm²(20mm X10mm) and shall be parallel to the longitudinal axis of the steel, located halfway from the outer surface of the steel to the center.

If the product standard does not specify, the sampling method is as follows:

Steel rods or billets with a diameter or side length greater than 40mm: the inspection surface is the partial radial section from the outer surface of the steel to the center.

3. principle

The observed test site was compared with the standard spectrum, and each type of inclusion was rated.

These rated images are equivalent to a square field of view with an area of 0.50mm² on a longitudinally polished plane 100 times lower.

According to the morphology and distribution of inclusions, the standard rating chart is divided into five categories: A, B, C, D and DS.

These five categories of inclusions represent the most commonly observed types and forms of inclusions:

Class A (sulfides) : A single gray inclusion with high ductility and a wide range form ratio (length/width), generally with rounded ends.

Class B (alumina) : most undeformed, angular, small shape ratio (generally <3), black or bluish particles, arranged in a line along the rolling direction (at least 3 particles).

Class C (silicates) : A single black or dark gray inclusion with high ductility and a wide range form ratio (generally ≥3), with an acute Angle at the end.

Class D (spherical oxides) : non-deformed, angular or round, small shape ratio (generally <3), black or bluish, irregularly distributed particles.

Class DS (single particle spherical class) : round or nearly round, diameter ≥13μm of single particle inclusion.

Each type of inclusion is divided into two series of fine and coarse according to the different width of non-metallic inclusion particles.

4. Determination of inclusion content

The sample was magnified 100 times under the microscope, and the inclusion grade was evaluated in the field of view of a square with a side length of 71mm and an actual area of 0.5mm².

Method A is used for production inspection

The entire finish should be inspected. For each type of inclusion, the number of levels of the standard picture corresponding to the worst field of view on the examined surface is recorded by fine and coarse series.

For individual inclusions whose length exceeds the side length of the field of view (0.71mm), or whose width or diameter is greater than the maximum of the coarse system (see Table 2), they should be assessed as super-size inclusions and recorded separately. However, these inclusions should still be included in the rating of the field of view.

For strip inclusions that are not in a straight line, if the longitudinal distance between the two inclusions is less than or equal to 40μm and the transverse distance along the rolling direction is less than or equal to 10μm, it shall be regarded as an inclusion.

If the widths of inclusions within an inclusion are different, the maximum width of the inclusion shall be regarded as the width of the inclusion.

If the inclusion length is too long or the width is too large, it should be recorded separately and included in the field of view for rating (the length of the extra long is 0.71mm).

5. Inclusion grade calculation (measurement rating picture grade of inclusions above 3)

Class A sulfide (length L expressed by μm) : Lg(i)=[0.560 5lg(L)]-1.179

Class B alumina: Lg(i)=[0.462 6lg(L)]-0.871

Class C silicate: Lg(i)=[0.480 7lg(L)]-0.904

Class D spherical oxides, where n is the amount in each field of view: Lg(i)=[0.5lg(n)]-0.301

DS class single particle spherical oxide, d is diameter: i=[3.311g(d)]-3.22

CANGZHOU GALAXY STEEL PIPE CO., LTD

sale@galaxy-steel.com

 

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Website: 

 https://www.friend-steelpipes.com/ 

 https://www.cz-steelpipe.com/

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ASTM A333 GR.6 seamless pipes, ( 1” to 24” sch40, sch80, sch160),

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Various structures and inclusions in steel (Chapter Two)

6 Bainite

The phase transition product formed between 350℃ and 550℃ of supercooled austenite is called the bainite structure, with the grain boundary as the axis of symmetry, and is like a feather. This feather structure is composed of parallel acicular or lamellar ferrite and short strip cementite distributed between the ferrite, the direction of the short strip cementite is roughly parallel to the lamellar ferrite. The strength of upper bainite is higher than that of pearlite formed from steel of the same composition. The upper bainite hardness is between 40-45 HRC.

7 Lower bainite

Below 350℃, the acicular structure formed between Ms points and above is called lower bainite. Lower bainite is a needle ferrite matrix with very fine carbide sheets distributed on it. In the crystal is needle-like, both ends pointed, needles basically do not cross, but can be exchanged, and tempered martensite is not easy to distinguish. High-carbon high-alloy needles are relatively thin, blue-black in color, and low-carbon low-alloy steel is gray.

Compared with upper bainite, lower bainite not only has higher strength, hardness and wear resistance, but also has good plasticity and toughness. The strength of lower bainite is similar to that of tempered martensite at corresponding temperature, and the hardness is between 45 and 55HRC.

8 granular bainite

The bulk ferrite contains some cementite particles and "islet" structures. It usually appears in low carbon and low alloy.

9 martensite

Martensite is a supersaturated solid solution of carbon in α-Fe.

When the carbon content of steel is low, when the steel is rapidly cooled from the austenitic state, it is transformed into a sheet martensite in the continuous cooling process, also known as low carbon martensite. Lath martensians are thin wooden strips arranged parallel to each other on a crystal face.

Lath martensite has higher strength and toughness, higher fracture toughness and lower cold brittle transition temperature.

High-carbon austenite forms sheet martensite, also known as acicular martensite. The martensite grows into the austenite crystal to form needles, and the needles are at a certain Angle, intersecting each other but not passing through. Martensite in steel has high strength and high hardness, but the plasticity and toughness are very low and the brittleness is large. The strength and hardness of martensite increase with the increase of carbon content in martensite, but the ductility and toughness decrease sharply.

10 Tempered martensite

When martensite is tempered at low temperature (150 ~ 250℃), most of the suoversaturated carbon is precipited from the inside of martensite in the form of highly dispersed cementite and carbide, so that martensite sheet is vulnerable to corrosion during the preparation of metallography sample, and appears black under the microscope, and carbide particles cannot be distinguished under the optical microscope. Such martensite is called tempered martensite. Tempered martensite retains the directionality of quenched martensite needles. In terms of performance, tempered martensite still has high hardness and high wear resistance, while reducing the internal stress and brittleness of quenching. Tempered hardness is generally 58-64HRC.

11 Temper troostite

The tissue obtained after tempering martensite at medium temperature (350℃ ~ 500℃) is called tempered troostite. Tempered troostite is a mixture of ferrite and cementite, in which cementite is fine granular distribution, martensitic needle direction is obvious, dark color, can not distinguish carbide particles at 500 times.

Tempered troostite has high elastic limit, yield limit and high toughness and plasticity. Medium temperature tempering is mainly used for various springs and hot dies, and the hardness after tempering is generally 35-50HRC.

12 tempered sorbite

The product of martensite tempering at high temperature (500℃ ~ 650℃) is called tempered sorbite. Tempered sorbite is also a mixture of ferrite and cementite, in which the cementite particles are coarser than those in tempered troosite and are clearer under metallographic microscopy. The purpose of getting tempered sorbite is to obtain good comprehensive mechanical properties of strength, plasticity and toughness. Hardness after tempering is generally 20-35 HRC.

Email: sale@galaxy-steel.com

 

Wechat: LXF13931739696.  Whatsapp: 008613931739696

 

Website:  https://www.friend-steelpipes.com/  https://www.cz-steelpipe.com/

 

CANGZHOU GALAXY STEEL PIPE CO., LTD

 

China Reliable Steel Pipe and Pipe Fittings Manufacturer and Supplier

 

My company hot sale steel pipe and pipe fittings:

 

ASTM A106 B seamless pipes,（1” to 24” sch40, sch80),

API 5L GR.B steel pipes, (seamless pipe, LSAW pipe, SSAW pipes, 1” to 110” sch 40, sch80),

Boiler tubes,( 20G. 12Cr1MoVG, ASTM A335 P5, P9, P11),

ASTM A333 GR.6 seamless pipes, ( 1” to 24” sch40, sch80, sch160),

TP304/304L, TP316/TP316L, stainless steel pipes, (1” to 48” sch10, sch20, sch40, sch80),

API 5L X65, X70 PSL2 LSAW welded pipes, (20” to 60” sch40, sch80, sch160),

API 5CT seamless casing, and tubing, (2 3/8” to 20” K55, J55, N80, L80, P110),

Pipe fittings, elbow, reducer, ,tee,cap, flange