NDT in Forging

NON DESTRUCTIVE TESTING CONCRETE IN CHENNAI, NDT EQUIPMENT SUPPLIERS IN CHENNAI, NDT EQUIPMENT MANUFACTURERS IN CHENNAI, NDT EQUIPMENTS IN CHENNAI, NDT EQUIPMENT, NDT TESTING EQUIPMENT IN CHENNAI, NDT TEST EQUIPMENT IN CHENNAI, NDT EQUIPMENT SALES IN CHENNAI, NDT EQUIPMENT FOR SALE IN CHENNAI, USED NDT EQUIPMENT FOR SALE IN CHENNAI, NDT EQUIPMENT RENTAL IN CHENNAI

In forging of both ferrous and nonferrous metals, the flaws that occur most often are caused by conditions that exist in the ingot, by subsequent hot working of the ingot or the billet, and by hot or cold working during forging. The non destructive Testing (NDT) methods most commonly used to detect these flaws include Visual, Magnetic Particle, Liquid Penetrant, Ultrasonic, Eddy Current, and Radiographic Inspection. This article discusses the applications of these methods to forging.

Flaws Originating in the Ingot

Many large open-die forgings are forged directly from ingots. Most closed-die forgings and upset forgings are produced from billets, rolled bar stock, or pre forms. Many, though by no means all, of the imperfections found in forgings can be attributed to conditions that existed in the ingot, sometimes even when the ingot has undergone primary reduction prior to the forging operation. Some, but again by no means all, of the service problems that occur with forgings can be traced to imperfections originating in the ingot.

  1. Chemical Segregation. The elements in a cast alloy are seldom distributed uniformly. Even unalloyed metals contain random amounts of various types of impurities in the form of tramp elements or dissolved gases; these impurities are also seldom distributed uniformly. Therefore, the composition of the metal or alloy will vary from location to location. Deviation from the mean composition at a particular location in a forging is termed segregation.
  2. Ingot Pipe and Centerline Shrinkage. A common imperfection in ingots is the shrinkage cavity, commonly known as pipe, often found in the upper portion of the ingot. Shrinkage occurs during freezing of the metal, and eventually there is insufficient liquid metal near the top end to feed the ingot. As a result, a cavity forms, usually approximating the shape of a cylinder or cone--hence the term pipe.
  3. High Hydrogen Content. A major source of hydrogen in certain metals and alloys is the reaction of watervapor with the liquid metal at high temperatures. The watervapor may originate from the charge materials, slag ingredients and alloy additions, refractory linings, ingot molds, or even the atmosphere itself if steps are not taken to prevent such contamination. The resulting hydrogen goes into solution at elevated temperatures; but as the metal solidifies after pouring, the solubility of hydrogen decreases, and it becomes entrapped in the metal lattice. Hydrogen concentration in excess of about 5 ppm has been associated with flaking, especially in heavy sections and high-carbon steels.
  4. Nonmetallic inclusions, which originate in the ingot, are likely to be carried over to the forgings, even though several intermediate hot-working operations may be involved. Also, additional inclusions may develop in the billet or in subsequent forging stages. Most nonmetallic inclusions originate during solidification from the initial melting operation. If no further consumable remelting cycles follow, as in air-melted or vacuum-induction products (with no remelting cycle to follow), the size, frequency, and distribution of the nonmetallic inclusions will not be altered or reduced in size or frequency during further processing. If a subsequent vacuum-remelting operation is used, the inclusions will be lessened in size and frequency and will become more random in nature. If an electroslag-remelting cycle is used, a more random distribution of inclusions will result.

Two kinds of nonmetallic inclusions are generally distinguished in metals:

  • Those that are entrapped in the metal inadvertently and originate almost exclusively from particles of matter that are occluded in the metal while it is molten or being cast.
  • Those that separate from the metal because of a change in temperature or composition Inclusions of the latter type are produced by separation from the metal when it is in either the liquid or the solid state. Oxides, sulfides, nitrides, or other non-metallic compounds form droplets or particles when these compounds are produced in such amounts that their solubility in the matrix is exceeded.
  • Unmelted electrodes and shelf are two other types of ingot flaws that can impair forgeability. Unmelted electrodes are caused by chunks of electrodes being eroded away during consumable melting and dropping down into the molten material as a solid. Shelf is a condition resulting from uneven solidification or cooling rates at the ingot surfaces.

Flaws Caused by the Forging Operation

Flaws produced during the forging operation (assuming a flaw-free billet or bar) are the result of improper setup or control. Proper control of heating for forging is necessary to prevent excessive scale, decarburization, overheating, or burning. Excessive scale, in addition to causing excessive metal loss, can result in forgings with pitted surfaces. The pitted surfaces are caused by the scale being hammered into the surface and may result in unacceptable forgings. Severe overheating causes burning, which is the melting of the lower melting point constituents. This melting action severely reduces the mechanical properties of the metal, and the damage is irreparable. Detection and sorting of forgings that have been burned during heating can be extremely difficult. In many cases, the flaws that occur during forging are the same as, or at least similar to, those that may occur during hot working of the ingots or billets.

Internal flaws in forgings often appear as cracks or tears, and they may result either from forging with too light a hammer or from continuing forging after the metal has cooled down below a safe forging temperature. Bursts, as described above, may also occur during the forging operation.

A number of surface flaws can be produced by the forging operation. These flaws are often caused by the movement of metal over or upon another surface without actual welding or fusing of the surfaces; such flaws may be laps or folds.

Cold shuts often occur in closed-die forgings. They are junctures of two adjoining surfaces caused by incomplete metal fill and incomplete fusion of the surfaces.

Surface flaws weaken forgings and can usually be eliminated by correct die design, proper heating, and correct sequencing and positioning of the work pieces in the dies.

Shear cracks often occur in steel forgings; they are diagonal cracks occurring on the trimmed edges and are caused by shear stresses. Proper design and condition of trimming dies to remove forging flash are required for the prevention of shear cracks.

Other flaws in steel forgings that can be produced by improper die design or maintenance are internal cracks and splits. If the material is moved abnormally during forging, these flaws may be formed without any evidence on the surface of the forging.

Selection of Inspection Method

The principal factors that influence the selection of an NDI method for forgings include degree of required integrity of the forging, metal composition, size and shape of the forging, and cost. There are sometimes other influential factors, such as the type of forging method used.

For high-integrity forgings, it is often required that more than one inspection method be employed because some inspection methods are capable of locating only surface flaws; therefore, one or more additional methods are required for locating internal flaws. For example, many forgings for aerospace applications are inspected with liquid penetrants (or with magnetic particles, depending on the metal composition) for locating surface flaws, then by ultrasonics for detecting internal flaws.

Certain characteristics or conditions unique to forgings can create service problems, yet these conditions are not easily detected by non destructive Testing. Exposed end grain, which can lead to poor corrosion resistance or to susceptibility to stress-corrosion cracking, is the most prevalent of the undesirable conditions. When certain steels or nonferrous alloys are forged at too high a temperature or sometimes when a part cools too slowly after forging, there is a potential for grain size in the finished forging to be excessively large. Such a condition is difficult to detect non destructively, except with ultrasonics, and then only when the grains are very large. Even with very large grains, ultrasonic inspection cannot determine grain size quantitatively, nor can it detect large grains reliably. Only the possibility that large grains are present can be inferred from excessive attenuation of the ultrasonic beam.

Non Destructive Testing

Visual Inspection

Despite the many sophisticated inspection methods available, unaided visual inspection is still important and is often the sole method of inspecting forgings used for common hardware items. Under proper lighting conditions, the trained eye can detect several types of surface imperfections, including certain laps, folds, and seams. Visual inspection is often used first, then questionable forgings are further examined by macroetching and inspection with macrophotography or some type of non destructive method. The only equipment necessary for visual inspection is a bench on which to place the forging and suitable cranes or hoists for forgings that are too heavy to lift by hand. Good and well-controlled lighting conditions are essential. Optical aids such as magnifying glasses that can magnify up to about ten diameters are often used to increase the effectiveness of visual inspection.

Magnetic Particle Inspection

Magnetic particle inspection is useful for detecting surface imperfections as well as certain subsurface imperfections that are within approximately 3 mm of the surfaces in forgings of steel, some grades of stainless steel, and other ferromagnetic metals. Magnetic particle inspection can be used with fluorescent particles and ultraviolet light.

The advantages of magnetic particle inspection include the following:

  • Almost instant results can be obtained in locating surface and certain subsurface imperfections.
  • Equipment can be transported to the forging, or the forging can be transported to the inspection station, as dictated by the size and shape of forging.
  • Preparation of the forging is minimal, mainly involving the removal of surface contaminants that would prevent magnetization or inhibit particle mobility.
  • Routine inspection work can be effectively done by relatively unskilled labour properly trained in interpretation.
  • For forgings that are simple in configuration, and when justified by the quantity, magnetic particle inspection can be automated.
  • For some forgings, electronic sensing can be used, thus reducing the chances of human error and increasing inspection reliability.
  • Many forgings have sufficient retentivity to permit the use of multidirectional magnetization, thus permitting the inspection of indications in all orientations with a single preparation. Retentivity must be checked for the particular forging before a decision is made to use multidirectional magnetization.
  • The cost of magnetic particle inspection is generally lower than that for several other inspection methods in terms of investment in equipment, inspection materials, and inspection time.

The limitations of the magnetic particle inspection of forgings are generally the same as for inspecting other work pieces and include the following:

  • The method is applicable only to forgings made from ferromagnetic metals
  • Because magnetic particle inspection is basically an aided visual inspection, under most circumstances, its effectiveness is subject to the visual acuity and judgment of the inspector
  • Magnetic particle inspection is generally limited to detecting imperfections that are within about 3 mm of the surface of the forging.
  • Because the forging must be thoroughly magnetized, magnetic particle inspection is likely to be ineffective unless scale, grease, or other contaminants are removed from the forging. Such surface contaminants inhibit the mobility of the particles necessary to delineate the indications.
  • Following inspection, the forging usually must be demagnetized, depending mainly on the retentivity of the particular metal, subsequent shop operations, and end use.

Liquid Penetrant Inspection

Liquid penetrant inspection is a versatile NDT process and is widely used for locating surface imperfections in all types of forgings, either ferrous or nonferrous, although it is more frequently used on nonferrous forgings. There is no limitation on the size or shape of a forging that can be liquid penetrant inspected. Any of the three basic liquid penetrant systems (water-washable, postemulsifiable, and solvent-removable) can be used to inspect forgings. The product or product form is not a principal factor in the selection of a system.

Advantages. Among the advantages of liquid penetrant inspection of forgings are the following:

  • There are no limitations on metal composition or heat-treated condition.
  • There are no limitations imposed on the size or shape of the forging that can be inspected.
  • Liquid penetrant inspection can be done with relatively simple equipment.
  • Training requirements for inspectors are minimal.
  • Inspection can be performed at any stage of manufacture.
  • Liquid penetrant materials can be taken to the forgings or the forgings taken to the inspection station, depending on the size and shape of the forgings.

The limitations of the liquid penetrant inspection of forgings are basically the same as those for the inspection of other work pieces. The characteristics of the surface of a forging sometimes impose specific limitations. The most important general limitations are:

  • Liquid penetrant inspection is restricted to detecting discontinuities that are open to the surface.
  • Liquid penetrant inspection is basically a visual aid; therefore, results depend greatly on the visual acuity and judgment of the inspector.
  • Satisfactory inspection results require that the surface of the forging be thoroughly cleaned before inspection. The presence of surface scale can cause inaccurate readouts. If the surface of the forging is excessively scaled, it should be pickled or grit-blasted, preferably pickled. The forgings should also be cleaned to remove surface contaminants, such as grease and oil
  • Liquid penetrant inspection is slower than magnetic particle inspection.

Ultrasonic Inspection

Ultrasonic inspection is used to detect both large and small internal flaws in forgings. Forgings, by their nature, are amenable to ultrasonic inspection. Both longitudinal or shear wave (straight or angle beam) techniques are utilized. The size, orientation, location, and distribution of flaws influence the selection of technique and the inspection results. Flaw orientation parallel to the sound beam will have no response in ultrasonic testing. There are, however, some definite limitations. All ultrasonic systems currently in use generate sound electrically and transmit the energy through a transducer to the orging. Because the relationship of sound transmitted to sound received is a factor in the inspectability of a forging, particular attention must be given to the surface condition of the forging. Although techniques and couplants can enhance the energy transmission from the transducer to the forging, as-forged surfaces impair the effectiveness of ultrasonic inspection. Near-surface flaws are most difficult to detect, and a dead zone at the entry surface often interferes. Because of the difficulty involved in detecting surface flaws by ultrasonic inspection, another method, such as magnetic particle or liquid penetrant inspection, is often used in conjunction with ultrasonic inspection to inspect high-integrity forgings thoroughly.

Complex shapes are difficult to inspect ultrasonically because of the problems associated with sound-entry angle. Most ultrasonic inspection of forgings uses techniques that send waves into the forging perpendicular to the surface. Radii, fillets, and similar configurations must receive special treatment if all areas of the forging must be inspected. The special treatment involves the use of a standoff that has an end contoured to fit the inspection surface or the use of a small diameter or focused transducer.

Application. In certain cases, where the end use of a forging is considered critical, ultrasonic inspection is used to inspect the wrought material before it is worked. Surface or internal flaws that are not detected before a billet is forged may not be detected in the final forging and will therefore be present in the finished part. Ultrasonic inspection is often used as part of a completely diagnostic inspection of a forging from newly designed dies, where use of the finished part does not warrant inspection of every part. Quality control measures often include the ultrasonic inspection of random samples from a particular forging. This provides the necessary assurance that the process is under control and that variables affecting internal quality have not been inadvertently introduced.

Ultrasonic inspection is often used in the further evaluation of flaws detected by other non destructive methods. This reduces the possibility that a particular forging will be unsuitable for its intended service.

Ultrasonic inspection can be used on every forging to validate its integrity for extremely rigorous requirements. This applies in particular to forgings for nuclear and aerospace applications, where rigid standards of acceptance have been established. Standards and criteria have been set up to detect material inclusions, internal voids, laminations, and other conditions. In addition, the inspection of every forging by ultrasonics has been effective in detecting excessive grain size and other structural conditions. Ultrasonic inspection is often used to qualify a particular lot of forgings that has been subjected to certain variations in approved processing procedures. A notable instance is the use of ultrasonics to determine the presence of thermal flakes or in locating quench cracks.

Eddy Current and Electromagnetic Inspection

In the non destructive inspection of forgings, the eddy current method is commonly used to detect flaws, while the electromagnetic method is used to detect differences in microstructure, chemical composition, or hardness. Electromagnetic inspection, which is restricted to ferromagnetic materials, is sometimes categorized as a modification of the eddy current method because both techniques are based on electromagnetic principles. In concept, eddy current equipment functions by the introduction of relatively high-frequency alternating currents into the surface areas of conductive materials. The response of the material to the induced field is then measured by a mechanism sensitive to the induced field.

Detection of Flaws

The detection of flaws in forgings by eddy current inspection is almost always done with a system consisting of a single probe that is connected to an instrument generically known as a defectometer. This system is balanced with the probe in air and is further balanced to a null value on sound material of the same composition, heat treatment, and surface condition as the forging to be inspected. Areas of the surface of the forging where flaws are suspected are scanned with the probe, which searches for an unbalance due to the flaw. Generally, the scanning is done in two directions approximately at right angles to each other.

The advantages of eddy current inspection for flaw detection include the following:

  • The unbalance level can be adjusted and calibrated with notches of known depth in the same material in the same condition, which can give a reasonable estimate of the depth of the flaw. This estimate can behelpful in reaching a decision as to the serviceability of the forging
  • In the event that flaws are oriented in one direction only, as with seams or rolling laps in the originalstock, the technique can be automated
  • Threshold gates, which are automatic signal-monitoring networks, are available to automatically signal flaws of a sufficient magnitude to be judged defective. These signals can in turn be used to mark or otherwise identify flawed forgings or locations on the forging having flaws
  • The automated tester can be operated by unskilled personnel once the system has been calibrated. Solid state electronics and their adaptation to eddy current equipment permit very stable instrumentation with no need for constant adjustment.

The disadvantages of eddy current inspection for detection of flaws include the following:

  • The correlation between unbalance signal and flaw depth is often not linear, for a variety of reasons. Accordingly, this method is frequently used as a go/no-go device at a depth of flaw, plus or minus a band of uncertainty. The band of uncertainty must be determined by experimental methods
  • In ferrous materials, variations in the decarburization levels can render the method invalid
  • This method is not suited to the detection of deep subsurface flaws

Detection of Differences in Microstructure

Differences in microstructure, which usually register as differences in hardness, can be detected by electromagnetic inspection using either encircling coils or spot probes as pickups. Regardless of whether a probe or a coil is used, the instrumentation must be set up and balanced in accordance with the manufacturer's directions. This is done without forgings in or near the pickup, with forgings in the pickup, or sometimes both with and without forgings. Once the test setup has been established, it is necessary to have good-quality forgings of known electromagnetic response available to ensure that the instrumentation has not varied. These forgings (which are sometimes referred to as masters) must be available to personnel who are trained in the setup and maintenance of the equipment.

The advantages of electromagnetic inspection for detecting differences in microstructure include the following:

  • The equipment can be electronically gated based on the response of the instrumentation to the properties of the forging
  • Electromagnetic inspection can be readily automated for properties throughout a forging (usually an encircling coil) or for properties at a specific location (using a spot probe)
  • Once it has been properly set up, the operation can be effectively run by unskilled personnel

The disadvantages of electromagnetic inspection for detecting differences in microstructure include the following:

  • A given response can indicate more than one condition in the forging; for this reason, testing technique must be developed very carefully. For example, an electromagnetic S-curve for a medium-carbon steel forging, when two variables (hardness and microstructure) exist, the curve shows essentially the same height in power loss for 37 HRC as for 56.5 HRC.
  • Development of techniques can be done only by trained personnel; even then, a great deal of experimentation is usually required to develop procedures that will yield accurate results.

Radiographic Inspection

Radiography (γ-ray or x-ray) is not extensively used for the inspection of forgings for two reasons. First, the types of discontinuities most commonly located by radiography (gas porosity, shrinkage porosity, and shrinkage cavities) are not usually found in forgings. Second, for the types of internal discontinuities that are commonly found in forgings (inclusions, pipe, bursts, or flakes), ultrasonic inspection is more effective, more adaptable, and more economical.

Radiographic techniques can sometimes be helpful in the further investigation of known internal discontinuities in forgings when the presence of these discontinuities has been determined earlier by ultrasonic inspection. In sections that are not too thick to penetrate with available radiographic equipment, the size, orientation, and possibly the type of discontinuities can be evaluated by radiography.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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