In ultrasonic testing of forgings, the variation of bottom wave amplitude is very important for evaluating the quality of forgings. Related to the reduction of bottom waves are not only individual defects and dense zone defects but also shrinkage cavities, severe segregation, crystalline pores, and small dispersed inclusions left due to insufficient forging ratio. If the reduction of the bottom wave is limited to the presence of individual defects and dense defects, it will result in the reduction of the bottom wave being unable to cover the evaluation of defects that affect the mechanical properties of the material, resulting in a decrease in the actual acceptance quality requirements.
In work, the author encountered: when the ultrasonic inspection of forgings is carried out according to the operation methods and defect determination rules stipulated in the current standards when the results of the ultrasonic inspection are judged to be qualified, the fracture failure accident will still occur in the unfinished manufacturing and trial use. The ultrasonic retest of the failed parts still needs to be qualified. There is no obvious single defect reflection wave in the detection area and no dense defect reflection wave. When the detection sensitivity is improved to the clutter sensitivity, the reflection of the flat bottom hole lower than φ2 is found many times. When the sound path is greater than a certain amount, the inherent clutter of the instrument and the material clutter of the workpiece to be tested cannot be distinguished. It is difficult to extract a single reflection larger than φ2 flat bottom hole and 5 or more dense area defect reflections from the reflected signal. However, the bottom wave can still be seen in the long distance. Because the existence of the reduction of the bottom wave is defined as an accessory existence attached to the existence of a single defect and dense defects, the problem of failure of forgings with qualified ultrasonic testing in subsequent processing and trial use has occurred several times.
So, is the determination of the existence of bottom wave reduction necessarily related to the existence of individual defects and dense defects? There are two questions to clarify:
- (1) What is the objective defect situation that causes bottom wave reduction in the workpiece, and is the existence of bottom wave reduction positively related to the existence of individual defects and dense defects;
- (2) Is it reasonable to understand the provisions of the implementation standards and determine the reduction of bottom wave based on the existence of individual defects and dense area defects?
1. Types of forging manufacturing micro defects and requirements for Ultrasonic testing
Many manufacturing defects of forgings come from the steel billets’ original defects. For example, smaller steel ingots will form dendritic crystals during free crystallization, causing loose and small voids and cracks. The central part of a large steel ingot forms large dendritic looseness, crystal gap cracks, segregation between the core and surface layer, shrinkage cracks, sand holes, and sand inclusions in the surface layer due to the inability to crystallize freely. The microstructure of the external surface layer and the central core are significantly different. There are also inclusions left when the end is not cut enough.
Some internal defects are related to heating history or heat treatment, such as needle-like structure, coarse grains that cannot be quenched, and quenching cracks near the surface. The rapid decrease in temperature and hydrogen content during the crystallization of steel ingots can cause small microcracks (hydrogen white spots). Still, this type of hydrogen induced cracking is now very rare.
These small original defects in steel ingots cannot be fully pressed during the forging process due to the low forging ratio of the forgings. The defects caused by the crystallization process vary among steel grades with different chemical compositions, but the quality effects formed by forging are similar.
Due to the presence of porosity, small inclusions, and microcracks in the core structure of these forgings, their mechanical properties differ from those of the surface layer of the forgings. This is a judgment that Ultrasonic testing should grasp, which cannot be determined by whether the implementation quality of the destructive test is qualified or not. Defects such as looseness, inclusions, and fine cracks in forgings generate noise, increase material attenuation, and significantly reduce bottom reflection. It is difficult to distinguish as cast coarse crystal reflection that needs to be reserved enough for forging, but its effect on the strength of materials performance is the same.
When inspecting these forgings, selecting a 1MHz – 1.5MHz low-frequency probe based on different reflected clutter is beneficial for increasing the sound path and expanding detection. Still, individual defects with small diameters may also be missing.
2. Industry standards related to forging manufacturing in China and their requirements for manufacturing quality
Quality inspection standards are technical normative documents established based on providing quality services for product manufacturing and are implemented to ensure that forgings meet the quality requirements of manufacturing standards.
Firstly, it is necessary to determine the requirements of industry standards for the manufacturing quality of forgings.
In JB4726-2000, “Carbon and Low Alloy Steel Forgings for Pressure Vessels” (replacing JB4726-1994) and the new version NB/T 47008-2010, “Carbon and Alloy Steel Forgings for Pressure Equipment” (replacing JB4726-2000), the manufacturing quality requirements for forgings are as follows:
“The head and tail of the steel ingot used for forging shall be cut sufficiently to ensure that the forging is free of shrinkage cavity, severe segregation, and other defects. When the steel ingot or steel embryo is used for forging, the forging ratio of the main section shall not be less than 3 (Electro-slag remelting steel shall not be less than 2). When using rolled material forging, the forging ratio of the main section of the forging shall not be less than 1.6. Forgings shall be formed by Hot working on the press, forging hammer, or rolling mill, and the metal on the whole section shall be forged through and forged as close as possible to the shape and size of the finished parts. Internal defects: Forgings should be guaranteed to have no white spots
Compared to the manufacturing standards of similar components in the same year, the requirements for manufacturing quality have not changed. In JB4727-2000, “Low Alloy Steel Forgings for Low-Temperature Pressure Vessels” (replacing JB4727-1994), and in the latest version of NB/T 47009-2010, “Low Alloy Steel Forgings for Low Temperature Pressure Equipment,” this section of the clause remains the same and unchanged.
This means that from a manufacturing perspective, it is required to have no defects such as shrinkage, severe segregation, and no shortage of forging ratio.
The impact of manufacturing defects on the mechanical properties of materials mentioned above is not detectable by ultrasound. When destructive testing cannot be used, only the detectability that can be converted can be utilized. Study the impact of material defects on the decline of material mechanical properties and the correspondence between non-destructive testing results, find a corresponding rule for non-destructive testing quality and destructive testing, and use non-destructive testing methods to provide alternative characteristics for evaluation to determine whether the forging quality is qualified or not. This is the quality significance of the description of bottom wave reduction in relevant manufacturing standards.
The manufacturing standard points out that Ultrasonic testing shall be carried out according to JB4730 to detect such manufacturing defects. The Ultrasonic testing index for such material defects as “serious segregation” and “insufficient forging ratio” is the reduction of the bottom wave.
3. Regulations on the reduction of a bottom wave when the relevant industrial standards for Ultrasonic testing of domestic forgings are replaced
From the earliest JB3963-85 “Ultrasonic Testing of Pressure Vessel Forgings,” it can be seen that the first description of the reduction of the bottom wave is as follows:
Bottom wave reduction BG/BF (dB): The ratio of the first bottom wave height (BG) in the defect free zone to the first bottom wave height (BF) in the defect free zone. The reduction in bottom reflection caused by defects is expressed in dB.
The provisions of defect classification are attached to the defect classification. The defect types are divided into single defect, bottom wave reduction, and dense area defect. It is emphasized that “the grades in Table 1 to Table 3 should be used as independent grades.” This expression has been continued to this day.
It is worth noting that the term “defect” in the definition refers to material defects related to the reduction of bottom waves, which are not limited to single defects and dense area defects. And the expression reduction in a bottom wave caused by defects which are used again in subsequent articles, is based on an understanding of the technical level at that time, indicating that the evaluation of defect level here excludes the reduction in a bottom wave caused by system factors such as surface roughness, coupling, surface contact attenuation, material attenuation, etc. This is explained in more detail in the standard “Compilation Instructions.”
In the compilation instructions of JB3963, it is stated in the “Explanation of Relevant Articles” that: “Due to the influence of the detected defects, although the reflection height of the defects is sometimes relatively low, the reflection of the bottom wave is significantly reduced. For example, large defects inclined towards the detection surface and small defects clustered together (including white spots and inclusions). So at this point, using only the reflected wave height of the defect to evaluate the size of the defect poses a risk of underestimation. According to the relevant provisions of ASTM A388 and NDIS 2411 standards on bottom wave attenuation, this standard has decided to consider the bottom wave reflection attenuation caused by defects and use it as an indicator for evaluating defects.
It is clear here that the reduction of the bottom wave is an independent indicator for evaluating defects. The “defects” mentioned in the above provisions refer to “large defects inclined towards the detection surface and small defects clustered together (including white spots and inclusions),” as well as material porosity, coarse grains, etc., caused by insufficient forging ratio, rather than individual defects and dense area defects.
Through extensive review, we can also see the description in JB3963 about “the adoption of foreign standards”: this standard is mainly based on the relevant provisions in the American ANSI/ASTM A388-80 Standard Recommended Practice Method for Ultrasonic Testing of Large Steel Forgings and the Japanese NDIS 2411-80 Standard Classification Method for Ultrasonic Testing and Inspection Results of Carbon and Low Alloy Steels. What can be found now is:
In 184.108.40.206 of ASTM A388-80 (up to the current year 2016a version) Standard Operating Methods for Ultrasonic Inspection of Steel Forgings, the statement that “no area with a reduction of bottom reflection greater than a certain percentage of reference bottom reflection is allowed” is the article 5.3.3 of ASTM E114-80 (up to the current year 2016 version) Standard Operating Methods for Pulse Reflection Direct Beam Ultrasonic Testing by Contact process Comparing the relative attenuation or velocity changes of sound waves can reveal the characteristics of materials, indicating that the reduction of bottom waves is used to characterize material characteristics, rather than to evaluate the severity of individual defects and dense area defects.
These articles, as mature and uncontroversial technical expressions, have remained unchanged after various revisions, demonstrating the invariance of the evaluation of forging manufacturing quality.
Although JB3963 was developed concerning Japanese standards, it did not adopt Japanese thinking methods. In Japan, the indicators of material quality are mostly the enhancement of forest clutter, the increase of sound Attenuation coefficient, and the number of bottom wave drops.
This is the origin of the definition of bottom wave reduction that has been used for the first time in China.
When the JB3963 standard was replaced by JB4730-1994 “Non-destructive Testing of Pressure Vessels” and reset to JB/T 4730-2005 “Nondestructive Testing of Pressure Equipment,” the definition of bottom wave reduction remained unchanged until the current NB/T 47013-2015 “Nondestructive Testing of Pressure Equipment.” The definition has been rewritten as follows:
The reduction in bottom wave caused by defects, BG/BF (dB), is the ratio of the first bottom wave amplitude BG in the defect free and intact area near the defect to the first bottom wave amplitude BF in the defect area, expressed as the sound pressure level (dB) value.
The author believes that compared to the definition of bottom wave reduction in JB3963. The planning and organization of the text is the same here. The term “defect” used in the definition still refers to “large defects inclined towards the detection surface and small defects clustered together (including white spots and inclusions),” as well as material porosity caused by insufficient forging ratio, coarse grains without quenching, small inclusions, small gaps that cannot be pressed, microcracks, etc. Among them are but not limited to single and dense area defects. So, adding a limiting attribute of “caused by defects” emphasizes that if the bottom wave reduction is caused by material scattering attenuation, surface coupling, geometric shape, etc., it is not included in the evaluation.
When updating standards, if there is no clear redefinition, the same terminology and unchanged defect classification should be assumed to maintain the continuity of the original definition and technical content in the replaced standards.
During the upcoming replacement of the JB4730-94 version to the 2005 version, due to on-site disputes over the acceptance of large forgings, some people interpreted the term “defect” in terms of “bottom wave reduction caused by defects” and the term “defect” in the definition of “intact area near the defect” as referring to the existence of “single defect and dense area defect.” Due to this narrow definition interpretation, the reduction of bottom waves is attached to the condition that there must be a single defect and a dense area of defects. And it should be explicitly stated that the reduction of the bottom wave is a related phenomenon caused by the influence of single defects and dense defects, and the evaluation of the reduction of the bottom wave is not considered for the absence of single defects and dense defects. This narrow evaluation has continued to the present day with the application of NB/T 47013.3.
Due to the dispute over the evaluation and acceptance of bottom wave reduction for large forgings, and to simplify the dispute and achieve the purpose of qualified evaluation, this narrow defect definition was first applied to the defect evaluation of forgings. Although this farfetched explanation seemed to solve the acceptance problem of some large forgings with the insufficient forging ratio at that time, it needed clarification for the widespread implementation of future standards.
To clarify the technical dispute mentioned above. A senior person who has participated in the development of all series of standards such as JB3963, JB4730-94, and JB/T 4730-2005 once again clarified in the promotion and explanation of the implementation of JB/T 4730-2005 standards that the reduction of the bottom wave is an independent type of defect evaluation:
When inspecting forgings, the changes in the bottom echo are crucial for evaluating the quality of the forgings. Usually, when the defect echo is high, multiple repeated echoes exist. The bottom wave severely decreases or even disappears; it indicates a large area of defects parallel to the detection surface in the forging. If the defect echo and bottom echo are both low or even disappear, it indicates the existence of a large area of defects tilted to the detection surface in the forging or a large defect near the detection surface Sink; If densely interconnected defect echoes (usually relatively low) appear on the oscilloscope screen, and the bottom echo significantly decreases or disappears, it indicates the presence of dense defects in the forging.
In summary, it can be found that the reduction of bottom waves caused by defects is a very important factor in the quality index of forgings, especially in the latter two cases where evaluation cannot be based solely on single defects or concentrated area defects. Therefore, to ensure the quality of forgings and improve equipment reliability, this standard stipulates that reducing bottom waves caused by defects is an important parameter for evaluating the quality of forgings. Through comparison, it is easy to find that the explanation here is the same as the initial explanation in JB3965-94.
In addition, JB4730-94 was formulated with an additional clause compared to JB3963-85: “If the material attenuation of the workpiece has a significant impact on the detection effect, heat treatment should be carried out again.” In the technical dispute over the acceptance of large forgings before and after 2004, it was involved that the bottom wave caused by the insufficient forging ratio in large forgings severely decreased. It is clarified that the influence of white spots, small inclusions, and cast microstructure on sound beam attenuation cannot be solved by heat treatment. So this article was canceled when JB/T 4730-2005 was formulated.
In JB/T 4730-2005, the description of sound attenuation caused by materials has retained: “When calculating defect equivalent if the Attenuation coefficient of materials exceeds 4dB/m, correction shall be considered.” The “Attenuation coefficient” is recorded in the “defect record” item. However, the fact that the Attenuation coefficient has been recorded as the defect is ignored in the entry of “quality grading and grade evaluation.” This is a legacy of unresolved technical disputes at the time. When the material attenuation is greater than 4dB/m, or even much greater than 4dB/m, is it related to the evaluation of material performance? How to evaluate it? This legacy issue has just been resolved.
When the version was replaced by NB/T 47013.3-2015 ‘Nondestructive Testing of Pressure Equipment Part 3: Ultrasonic Testing’, some changes were made to the defect evaluation of forgings: the defect recording clause was canceled, and the attenuation coefficient was canceled. The classification requirement of defect equivalent diameter in the dense area is increased to φ2 flat bottom hole. No changes have been made to the definition of bottom wave reduction and dense area defects, defect classification, and evaluation level data.
The Ultrasonic testing of forgings did not refer to the ultrasonic testing method for macro defects (the current standard is GB/T 34018-2017 non-destructive testing ultrasonic microscopic testing method) to conduct a regular inspection for such material defects. Still, such macro defects due to manufacturing reasons may cause harm to the material, which cannot be ignored. Combined with the characteristics of conventional ultrasonic detection, such defects can still only be quantitatively evaluated through qualitative detection based on the reduction of bottom waves.
Due to the inconsistent interpretation of the standard by the standard setter, the evaluation of Ultrasonic testing has a vacuum area that is not covered, resulting in evaluation confusion that has yet to be seen in Ultrasonic testing. The conflict between the qualified Ultrasonic testing quality and the actual unqualified forging manufacturing quality falls on the staff of the testing front line.
4. Further verification of Ultrasonic testing results
The confusion of evaluation in ultrasonic testing caused by different standard interpretations described in Chapter 3 of this paper leads to the failure caused by decreased detection quality. According to the provisions of the independent ultrasonic testing by different people to review, different degrees of failure parts in the retest found no ultrasonic testing missed or misjudgment.
The reflection of φ2 flat bottom hole equivalent of the single defect and dense area defect determined by ultrasonic testing and the reflection of φ2 flat bottom hole equivalent on the screen when the reduction of the bottom wave is confirmed by ultrasonic testing are different.
This is reflected in the characteristics and anatomical morphology of ultrasound detection:
- (1) Single defect and dense defects ofφ2 flat bottom hole equivalent reflection, reflection depth is fixed. After the equivalent diameter of the flat bottom hole is given according to its wave height, drawing the boundary range is unnecessary. However, the half-wave height method can draw the diameter range. Even if the range overlaps, the boundary can be drawn. The size is close to the 6dB diameter of the sound beam at the sound path. The side observation after dissection is marked with a ruler. The depth is observed with the milling cutter, and the contents can be seen with or without the help of a magnifying glass and a microscope, and the statistics are scattered. The display is a single distribution; the single size is slightly larger than the ultrasonic equivalent size, which is similar, irregular in shape, and statistically dispersed.
- (2) When the reduction of the bottom wave occurs obviously at-2dB and can be identified stably, with the reflection of the equivalent φ2 flat bottom hole, the wave root is very wide on the time baseline, and the display depth of the reflected wave moves on the time baseline when the probe moves. When the half wave height method is used to draw its diameter range, it is difficult to draw ‘one, ‘ and the size of its continuous range becomes the continuous existence of the whole piece in the range of the bottom wave reduction.
- (3) In the anatomical observation of the specimen where the bottom wave is reduced, it is seen that the small dots are dispersed in a continuous or sometimes layered distribution, with the help of a magnifying glass or even 16 times to 32 times the microscope.
Compared to the longitudinal wave with a frequency of 2.5MHz and a sound speed of 5950m/s, it is only 1/12-1/5 of the wavelength, measured at 0.2mm-0.5mm. This does not comply with the classic theory described in the textbook that “when the size of these reflectors is less than 1/4 of the wavelength, sound waves will bypass without reflection”. However, it is similar to the situation in some foreign experiments, but due to limited conditions, accurate measurement data needs to be. This is the statistical result of multiple experiments conducted at different locations with multiple participants. The participants in the experiment unanimously believe that when theory and practice do not match, they should be loyal to the experimental data and correct the theory rather than practice.
The equivalent diameter of these reflected waves is positively correlated with the density of dispersed inclusions observed in metallographic experiments. We believe that in the area that significantly affects the reduction of bottom waves, the equivalent wave height of a certain flat bottom hole displayed on the instrument screen within a certain depth and width range is the collective sound pressure of each small reflector. In the specimen, it was also measured that there is a point where the collective sound pressure is equivalent to ϕ Reflected wave height equivalent to 5 flat-bottomed holes.
The author believes that the manifestation of this ultrasound characteristic and the visual objects observed after dissection are the reasons for the unqualified material characteristics mentioned in Chapter 1.
It is worth noting that in the subsequent mechanical tests that sampled the suspected area, there were also:
- (1) During the mechanical performance experiment, the yield strength ReL of this part (σs) was almost coincident with the tensile strength. And the tensile strength Rm (σb) will decrease to below the minimum value of the steel grade.During the processing of the test piece, experienced machining operators can determine that the material of the test piece has deteriorated from the debris falling from the processing.
- (2) V-notch impact toughness Ak of Charpy impact test（αk value will drop to almost zero.
- (3) During the flat milling process, when a 0.2mm cutting edge is milled through one layer, the original image is no longer visible but appears again in a new position. Continuous observation layer by layer gives a sense of trace migration.
- (4) There are also dual-phase steel forgings with a forging ratio of less than 1.5, whose ultrasonic characteristics are similar to the interface reflection waveform of the stainless steel surfacing layer. The size of the hairline-like inclusions in its metallographic experiment mostly reaches 1mm, with occasional indications of 2mm or more.
Analysis suggests that the middle of the forging is the final crystallization zone of the original steel billet, with some end inclusions extending into the depth of the billet. The sources of these inclusions and microcracks are the original inclusions and pores in the steel billet, which were not pressed during forging, resulting in larger hairline shapes related to the forging ratio.
The results of anatomical analysis draw a statistical conclusion. If there is no single defect or concentrated area defect, it is not necessary to check whether there is bottom wave reduction. The qualified ultrasonic testing quality obtained in this way cannot cover the qualified strength of materials performance quality.
Even if the defect assessment is now based on NB/T47013.3-2015, if the defect assessment of the bottom wave reduction adheres to the pre-condition of the existence of a single defect and a dense defect, the increase of φ2 flat bottom hole equivalent to the defect in the dense area cannot cover the assessment of the bottom wave reduction. Those loose areas or small inclusions with insufficient mechanical properties that have been omitted cannot meet the requirements of manufacturing standards for manufacturing quality. This is the difficulty of forging detection the author has encountered since 2005.
The following is a failure diagram of forgings during production due to decreased material mechanical properties, as shown in Figures 1 to 4.
Figure.1 Side view of the cracked part of the pressure bearing joint between the boom frames of a crawler crane (failed during production)
Figure.2 Cracks in the tail shaft shell (forging) of the 4650T project (failure during production)
Figure.3 Side view of the cracked part of the transmission link of a large forging press (failed during use)
Figure.4 Equipment flange (forging) of DN600 (broken during welding with head)
In NB/T47013.3, the equivalent diameter of the defect in the dense area is increased to the benchmark of φ2 flat bottom hole quantification, which is only for the defects in the dense area. Although the defects in the dense area sometimes span the small inclusion defects of the material, it cannot solve the problem that most bottom wave reductions do not cover the evaluation of material defects. This evaluation benchmark, due to the large depth of the workpiece to be inspected during operation, the limitation of the effective observation dynamic range of the instrument, and sometimes the material clutter, signal-to-noise ratio, and other reasons will make the detection of the maximum amplitude of the bottom reflection wave of the maximum sound path and the instrument. The dB difference between the clutter level does not meet the dynamic range used in instrumental analysis, and its detection sensitivity is also difficult to achieve. This is a realistic problem that cannot be ignored. However, it is still clear to observe the bottom wave reflection and its reduction degree on the maximum sound path. Therefore, it is unreasonable to replace the evaluation of the reduction of the bottom wave with the increase of the reflected wave equivalent of the defects in the dense area.
Even if individual and concentrated area defects are judged qualified, the quality of forgings remains insecure. The insecurity of this quality comes from material defects due to loose materials, microcracks in gaps, and densely dispersed inclusions, which reduce the mechanical properties of forgings. Some authors have encountered cast steel and counterfeit forgings, and some have no reflection on the screen; even the bottom wave cannot be seen. However, it cannot be judged as qualified because there are no reflection waves and can only be judged as unqualified forgings.
When the thickness of the workpiece to be inspected is 200mm, and the reduction of the bottom wave is 20dB, the Attenuation coefficient of the sound can be converted to 100dB/m. There is no single or dense defect in the entangled sound path, so it is meaningless to refuse to assess the reduction of the bottom wave.
The invalidation judgment of forgings qualified by Ultrasonic testing in subsequent processing undoubtedly increases the production cost, delays the manufacturing period, and causes losses to the factory. This situation occurs not only on small and easily replaceable component forgings but also on large forgings of pressure vessel pressure components.
After the failure, strict supervision and testing proved that the testing personnel’s testing and evaluation were faultless and irresponsible. However, the results of dissection or grinding confirm that there are visually visible defects in the manufacturing quality. This quality conflict phenomenon that Ultrasonic testing is qualified but manufacturing quality is unqualified must be addressed. Many people are not involved in the final acceptance of the owner’s products but in the quality control of production inspections during manufacturing. It is the responsibility of the enterprise to ensure that unqualified raw materials are not circulated downwards and that parts that can be processed to avoid or eliminate defects are not scrapped, which is a requirement for the inspection position during the process. Cost reduction and integrated use are two accounting boundaries; the distance between them is the Lebensraum of process inspection.
The formulation of standards is complex, as it is the result of compromises and coordination among multiple different opinions. Therefore, the use of standards should not be mechanical but based on a comprehensive understanding of the background of standard formulation from the perspective of standard formulation and the technical points of disputes and disagreements in coordinating compromises. Only in this way can standard provisions be appropriately used when there is no clear text or unclear understanding of the standards.
Finally, it is clarified that the reduction of bottom waves is one of the indicators for evaluating the quality of forgings. The significance of its existence lies in not being used as an additional assessment of the severity of individual defects and dense area defects but rather as a separate quality indicator for evaluating material properties.
Author: Liu Tao