A method for analyzing the fracture of high-cr steel or alloy cracked in high-temperature environment

Abstract

This invention discloses a method for analyzing the fracture surface of high-Cr steel or alloy steel that cracks in a high-temperature environment. The method involves cutting a metallographic sample at the crack tip of a crack-containing block, polishing it, visually observing the crack morphology, and analyzing the composition of corrosion products on the sample. The sample is then etched with an etchant, and the relationship between the crack direction and the microstructure of the sample is observed under a metallographic microscope to obtain the crack metallographic test results. The remaining crack in the crack-containing block is opened, and the macroscopic features of the fracture surface are visually observed. The microscopic features of different regions of the fracture surface are observed using a scanning electron microscope, and the composition is tested to obtain the micro-region composition. The surface corrosion of the fracture surface is removed using an electrostatic descaling method, and the microscopic features of different regions of the fracture surface are observed under a scanning electron microscope. Based on the crack metallographic test results, the macroscopic and microscopic features of the fracture surface, and the micro-region composition, the cause of the cracking is determined. This invention provides a complete analytical procedure, yields abundant information, and effectively analyzes the causes of component cracking.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of material failure cause analysis, and particularly relates to a high-temperature environment cracking high-Cr steel or alloy fracture analysis method. BACKGROUND

[0002] Cracking or even breaking of components is the most common failure form, and analyzing the cracking or breaking causes and proposing corresponding preventive measures can reduce or even eliminate the repeated occurrence of similar problems. The cracking initiation cause, direction and fracture characteristics of different regions are important means for analyzing the cracking development process and the failure mechanism of components, and therefore the fracture needs to be analyzed. Fracture analysis includes fracture macro and micro morphology analysis, fracture metallographic analysis and the like. The existing technology often only performs partial analysis, and there are problems such as imperfect analysis items and unreasonable steps, which lead to the difficulty in accurately obtaining the actual cracking cause of components.

[0003] The fracture surface, especially the fracture surface serving at high temperature, is often covered by corrosion products, which leads to the fact that the fracture morphology characteristics are covered and the micro mechanism of failure is difficult to accurately explain. Therefore, effective means need to be taken to clean the fracture before analyzing the micro characteristics of the fracture, and the surface corrosion products are removed without damaging the real metal morphology characteristics of the fracture.

[0004] The existing fracture cleaning methods mainly include: (1) organic solvents such as alcohol and acetone are used for immersion, brushing or ultrasonic cleaning to remove surface oil stains and dust, but cannot remove firmly adhered corrosion products. (2) AC paper or adhesive tape can be used to remove loose corrosion products, but the effect is not good for dense corrosion products, such as the oxide layer formed at high temperature. (3) Chemical etching liquid immersion method often uses acid liquid immersion to remove corrosion products, but because the metal matrix can also react with the acid liquid, it is relatively easy to clean excessively and damage the micro morphology of the fracture.

[0005] Increasing the content of Cr element is one of the most important means to improve the oxidation corrosion resistance of steel and alloy. High-Cr steel or alloy refers to iron-based, nickel-based or iron-nickel-based alloy and the like with a Cr content of more than 16%, which is widely used in the fields of electric power, aviation, aerospace and the like. Because of the high Cr content, it has good oxidation corrosion resistance, but because Cr has strong affinity with O, the bare metal will be quickly oxidized at high temperature when serving in an oxidizing environment such as high-temperature steam, air, flue gas or fuel gas, and a dense oxide layer mainly composed of Cr2O3 will be formed on the surface. Therefore, when the high-Cr alloy serving at high temperature cracks, an oxide layer mainly composed of Cr2O3 will be quickly formed on the fracture surface, covering the real morphology of the fracture surface, thereby bringing difficulties to the analysis of the failure cause of the component.

[0006] Patents CN 110528010 B, CN102418107A, CN110344066A, CN108796514A, CN109778203A, CN105839118A, etc. all use acid liquid to clean the fracture, because the metal matrix can react with hydrochloric acid, this method is easy to cause excessive cleaning to damage the fracture micro morphology. CN101750243B uses AC paper to paste the fracture, which can only remove loose corrosion products formed at room temperature, and has poor effect on dense corrosion products formed at high temperature. SUMMARY

[0007] The purpose of the present application is to overcome the shortcomings of the prior art, provide a high-temperature environmental cracking high-Cr steel or alloy fracture analysis method which is perfect in analysis steps, reasonable in arrangement, complete in fracture scale removal, and does not damage the metal matrix.

[0008] The present application is realized by the following technical scheme:

[0009] A high-temperature environmental cracking high-Cr steel or alloy fracture analysis method, comprising the following steps:

[0010] (1) Use electric spark cutting or mechanical cutting method to cut a block containing most of the cracks from the high-temperature environmental cracking high-Cr steel or alloy, to obtain a crack-containing block;

[0011] (2) Use electric spark cutting to prepare a crack-containing metallographic sample at the crack tip of the crack-containing block, to obtain a crack tip metallographic sample, polish and polish the plane of the crack tip metallographic sample, first observe the crack morphology in the polished state and take a photo, if necessary, analyze the composition of the corrosion product on the plane of the crack tip metallographic sample by using a scanning electron microscope equipped with an X-ray energy spectrum, then corrode the crack tip metallographic sample with an etchant, and observe the relationship between the crack direction and the microstructure of the crack tip metallographic sample by using a metallographic microscope, to obtain the crack metallographic test result;

[0012] (3) Use a hammer to knock or a three-point bending method to open the remaining cracks of the crack-containing block, to obtain two matching fractures;

[0013] (4) Preliminary cleaning of the fracture, soaking the fracture in an organic solvent for not less than 5 minutes to remove grease and dust on the surface of the fracture;

[0014] (5) After preliminary cleaning of the fracture, first visually observe the macroscopic characteristics of the crack, if necessary, observe the macroscopic characteristics of the fracture by using a stereomicroscope, and preliminarily find out the crack source area, expansion area and breaking area according to the surface color, deformation degree, herringbone, shell pattern, and radiation direction, etc.

[0015] (6) Put the fracture in scanning electron microscope to observe the micro features of different areas of the fracture, test and compare the compositions of the crack source area, the expansion area and the breaking area, and obtain the micro area composition analysis result;

[0016] (7) Take one side of the fracture, remove the corrosion products generated by the high temperature service on the surface of the fracture by electrolytic descaling method until the surface of the fracture presents metallic luster, then rinse with water and anhydrous ethanol and dry;

[0017] (8) Put the fracture after removing the surface corrosion products in scanning electron microscope to observe the micro features of different areas of the fracture again;

[0018] (9) If necessary, take the metallographic sample of the crack source area of the side of the fracture without removing the corrosion products on the surface of the fracture by wire cutting, the wire cutting plane is perpendicular to the fracture plane, observe the features of the crack source area after grinding and polishing the metallographic sample.

[0019] (10) Obtain the fracture cracking reason according to the crack metallographic test result, the macro features and micro features of the fracture and the micro area composition analysis result.

[0020] Preferably, in step (3), before opening the remaining cracks, the block containing the cracks is immersed in liquid nitrogen, and the remaining cracks are opened by knocking or three-point bending after sufficient cooling.

[0021] Preferably, in step (4), the organic solvent used for the preliminary cleaning of the fracture includes but is not limited to alcohol, acetone, gasoline and diethyl ether.

[0022] Preferably, in step (4), the preliminary cleaning of the fracture can use an ultrasonic cleaning machine to enhance the cleaning effect.

[0023] Preferably, in step (7), the electrolytic descaling method uses sodium chloride, sodium hydroxide and water to prepare an electrolytic descaling solution according to a ratio of 1:1:10.

[0024] Preferably, in step (7), the electrolytic descaling method connects the fracture with a cathode, uses stainless steel as an anode, and the current is 3-5 A and the voltage is 13-17 V.

[0025] Preferably, in step (2), the plane of the crack tip metallographic sample is perpendicular to the crack.

[0026] Preferably, the Cr content of the high Cr steel or alloy is more than 16%.

[0027] Compared with the prior art, the present application has the following beneficial effects:

[0028] The analysis steps of the present application are perfect and reasonable in arrangement, and rich information can be obtained to effectively analyze the cracking causes of the parts. First, before opening the crack, a metallographic sample is cut at the crack tip, the information of the crack propagation front can be obtained first without damaging the crack source area, and after cutting part of the crack front, the difficulty of opening the crack can be significantly reduced; the crack morphology is observed in a polished state, and the corrosion product X-ray energy spectrum analysis is carried out, the true information of the corrosion product can be obtained, and if the cracking is caused by the aggregation of non-metallic inclusions, corrosion and the like, it can be found as soon as possible to avoid the interference of the metallographic etchant. Secondly, after the fracture oil and dust are removed by preliminary cleaning, macroscopic observation is carried out first, the key observation areas such as the source area and the expansion area are preliminarily found out according to the fracture color, deformation, and trace direction, and then the micro-morphology and micro-area composition are observed by a scanning electron microscope, which can significantly improve the observation efficiency of the electron microscope and reduce the consumption of the expensive electron microscope, and the fracture surface information can be fully obtained before the degreasing and cleaning by the electron microscope observation, so as to avoid missing the information related to corrosion. After the fracture is opened, there are two sides, and only one side is electrolytically cleaned to obtain the actual micro-morphology information of the fracture. The corrosion product formed by the high-Cr steel or alloy in the high-temperature environment is often dense, and it is difficult to remove the scale layer by the conventional method, so the electrolytic method is adopted to remove the scale layer from the fracture. Therefore, the information of the cracking of the high-Cr steel or alloy part in the high-temperature environment can be fully obtained, and the most sufficient data guarantee is provided for the analysis of the failure causes of the part.

[0029] Further, the plane of the crack tip metallographic sample is perpendicular to the crack, which can make the crack length observed by the metallographic observation more consistent with the actual situation, and the relationship between the crack direction and the stress can be easily analyzed.

[0030] Further, the crack tip is immersed in liquid nitrogen to reduce the temperature before the crack is opened, which can significantly reduce the force required for opening the crack, and plastic deformation does not occur during the opening process to affect the fracture observation and analysis.

[0031] Further, in addition to visual observation, stereomicroscope is used for auxiliary observation, which can quickly observe the characteristics of the small area on the fracture and reduce the consumption of the expensive electron microscope.

[0032] Further, in the electrolytic descaling method, the fracture is connected with the cathode, so that the metal loss does not occur, the excessive cleaning is avoided, and the real micro-morphology of the fracture can be obtained.

[0033] Further, the metallographic sample of the crack source area is taken from the other side, so that the metallurgical characteristics of the crack source area can be obtained at the same time, and the information acquisition is more sufficient. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 It is a scanning electron microscope photo of the source area after electrolytic descaling in the embodiment 1 of the present application;

[0035] Figure 2The source region scanning electron microscope photograph after preliminary cleaning and before electrolytic descaling of Example 1 of the present application. DETAILED DESCRIPTION

[0036] In order to make the personnel in the technical field better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by the person of ordinary skill in the art without creative labor should belong to the protection scope of the present application.

[0037] It should be noted that the terms "first", "second" and the like in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily indicate a specific order or a chronological sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or units does not have to be limited to those steps or units clearly listed, but can include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0038] Example 1

[0039] The cracking component is a power station valve, the material is a nickel-based alloy with a Cr content of 23%, the normal working temperature is about 700℃, and the working medium is steam. First, an inclusion crack block containing most of the cracks is cut by electric spark cutting, a crack-containing metallographic sample is prepared at the crack tip of the inclusion crack block, a crack morphology is observed in a polished state, no non-metallic inclusions or other segregations are found near the crack, the composition of the corrosion product in the crack is analyzed by a scanning electron microscope equipped with an X-ray energy spectrum, it is found that the corrosion product is an oxide, the metal elements are all elements contained in the valve itself, no corrosive elements such as S and Cl are found, and the crack is observed to be a characteristic of intergranular corrosion after etching with an etchant. After immersion in liquid nitrogen, the remaining cracks are opened by a three-point bending method, two matching fracture surfaces are obtained, the fracture surfaces are preliminarily cleaned by acetone + ultrasonic wave to remove grease and dust on the surface of the fracture surfaces, the macroscopic characteristics of the cracks are observed, the possible source region of the cracks is found to be at the sealing surface according to the color of the surface of the fracture surfaces, the fracture surfaces are placed in a scanning electron microscope to observe the micro-features and micro-area composition of different regions of the fracture surfaces, it is found that the source region is covered by dense oxides, and no corrosive elements are found, the expansion region is a characteristic of intergranular corrosion, and the surface oxide layer is thin, as shown in FIG. 1. Figure 2The fracture surface is shown; take one side fracture, adopt electrolyte of sodium chloride: sodium hydroxide: water as 1:1:10 to electrolysis, remove the corrosion product of the fracture surface of the nickel-based alloy due to the high temperature service, until the fracture surface presents the metal luster, after washing with water and anhydrous ethanol in turn and drying, observe the fracture micro morphology under the scanning electron microscope again, find the fatigue characteristics in the fracture source area, such as the attached Figure 1 The fracture cracking reason is clear, and it is not necessary to prepare the source area metallographic sample again. According to the above results, it is judged that the cracking property is fatigue cracking, and after consulting the operation history of the component, it is found that the steam temperature changes too fast during the start and stop of the unit, and the cracking reason is thermal fatigue.

[0040] Example 2

[0041] The cracked component is a boiler superheater tube, the material is a Ni-Cr-Co-Mo alloy with a Cr content of 22%, the working temperature is about 680 DEG C, the inside of the tube is steam, the outside of the tube is boiler flue gas, and the cracking position is the outer wall of the tube. First, the crack-containing block including most of the cracks is cut by electric spark cutting, the crack tip of the crack-containing block is cut by electric spark cutting to prepare a crack-containing metallographic sample, the crack morphology is observed in a polished state, it is found that there are accumulated corrosion products in the crack and near the crack tip, the composition of the corrosion products in the crack is analyzed by a scanning electron microscope equipped with an X-ray energy spectrum, it is found that the corrosion products in the crack are Cr and O-rich oxides, and contain S; the accumulated corrosion products near the crack tip are Mo and S-rich, and it is found that the crack is a transgranular characteristic after observing after etching with an etchant; compared with a normal tube, the aging degree of the material metallographic structure is large. Fiber Vickers hardness tests are performed on different regions, and it is found that the hardness difference is large, and the suspected improper heat treatment. After immersion in liquid nitrogen and knocking, the remaining cracks are opened to obtain two matched fracture surfaces, the fracture surfaces are preliminarily cleaned by acetone + ultrasonic wave to remove the grease and dust on the fracture surfaces; the macroscopic characteristics of the cracks are observed, it is found that the entire fracture surface is covered with dense corrosion products, and the X-ray energy spectrum analysis shows that the crack is related to sulfur corrosion; because the crack surface is covered with too thick corrosion products, the fracture microscopic characteristics have been damaged by corrosion, and at this time, the cracking reason is clear, and it is not necessary to perform the steps of fracture cleaning and source area metallographic analysis. According to the above results, it is judged that the cracking reason is that the improper heat treatment of the material causes excessive stress, accelerates the sulfur corrosion of the outer wall, and causes cracking.

[0042] The high-temperature environment cracking high-Cr steel or alloy fracture analysis method of the application has perfect analysis items and reasonable step arrangement, and can effectively analyze the cracking reason of the component. The corrosion product formed by the high-Cr steel or alloy in the high-temperature environment is often dense, and it is difficult to remove the scale layer by the conventional method. The scale layer removal effect is good by adopting the alkali electrolysis method on the fracture surface, the fracture surface is connected with the cathode, metal loss does not occur, excessive cleaning is avoided, and the real micro morphology of the fracture surface can be obtained.

[0043] The above merely illustrates the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application on the basis of the technical scheme falls within the protection scope of the present application.

Claims

1. A method for fracture analysis of high Cr steel or alloy cracked in a high temperature environment, characterized by, The method comprises the following steps: cutting a block containing cracks from a high-Cr steel or high-Cr alloy cracked in a high-temperature environment to obtain a block containing cracks; cutting a crack tip metallographic sample at a crack tip of the block containing cracks, polishing and polishing a plane of the crack tip metallographic sample, visually observing a crack morphology after polishing, and analyzing a composition of corrosion products on the plane of the crack tip metallographic sample by using a scanning electron microscope equipped with an X-ray energy spectrum; then etching the crack tip metallographic sample with an etchant, observing a crack direction and a relationship between a microstructure of the crack tip metallographic sample after etching is completed by using a metallographic microscope, and obtaining a crack metallographic test result; wherein the plane of the crack tip metallographic sample is perpendicular to the crack; immersing the block containing cracks in liquid nitrogen, opening a remaining crack after cutting the crack tip metallographic sample after the block containing cracks is cooled, and obtaining two matched fracture surfaces; visually observing macroscopic features of the fracture surfaces after preliminary cleaning; observing microcosmic features of different regions of the fracture surfaces by using a scanning electron microscope and testing compositions of the different regions to obtain micro-area composition analysis results; taking one of the fracture surfaces, removing corrosion products on a surface of the fracture surface by using an electrolytic descaling method, rinsing and blowing dry, and placing the fracture surface under a scanning electron microscope to observe microcosmic features of different regions of the fracture surface; the electrolytic descaling method uses a mixed solution of sodium chloride, sodium hydroxide and water as an electrolytic descaling liquid; the electrolytic descaling method is to connect the fracture surface with a cathode, use stainless steel as an anode, and perform electrolysis; taking the other of the fracture surfaces which does not remove the corrosion products on the surface by using the electrolytic descaling method, cutting a crack source area metallographic sample from the fracture surface, and observing features of the crack source area after grinding and polishing the crack source area metallographic sample; obtaining a fracture cracking reason according to the crack metallographic test result, the macroscopic features and the microcosmic features of the fracture surfaces, and the micro-area composition analysis results.

2. The high temperature environmental cracking high Cr steel or alloy fracture analysis method according to claim 1, characterized by, The high-Cr steel or high-Cr alloy has a Cr content of 16% or more.

3. The high temperature environmental cracking high Cr steel or alloy fracture analysis method according to claim 1, characterized by, The organic solvent used for the preliminary cleaning of the fracture surfaces is alcohol, acetone, gasoline or diethyl ether.

4. The high temperature environmental cracking high Cr steel or alloy fracture analysis method according to claim 1, characterized by, In addition to visually observing the macroscopic features of the fracture surfaces after the preliminary cleaning, stereoscopic microscopy is also used to observe the macroscopic features of the fracture surfaces to find out a crack source area, an expansion area and a broken area.

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