|
Written by Gus
|
|
Friday, 13 August 2010 10:25 |
Leak Rates, Leak Detection and Leak Repair
by Daniel H. Herring
July 29, 2010
This is the 12th in a series of articles in our Vacuum Heat-Treatment Series. What follows is a discussion about acceptable leak rates, leak detection and leak repair methods used on most vacuum vessels. Controlling the leak rate is of major importance in producing sound metallurgical components.
A common problem experienced by almost every vacuum user is that, over time, leaks develop that are both damaging to product quality and to furnace internal components. In extreme cases, the problem is obvious: the furnace will not pump down and/or the hot zone (or heating elements) shows obvious signs of oxidation. Small leaks, which are more common, often go undetected because the pumping system can overcome any air infiltration. However, even small leaks can cause continuous and sometimes catastrophic damage. Thus, routine leak checking should become a part of any good vacuum furnace maintenance program. What is a Vacuum Leak? A leak is an opening such as a crack or hole that allows a substance to be admitted to or to escape from a confined space. A vacuum system leak allows air to enter into the vacuum vessel. Suspect areas on vacuum furnaces include threaded and brazed joints, fittings that have been improperly sealed or installed, and damage (e.g., cut, worn, melted or dirty O-ring seals, especially around doors). Components that rotate or reciprocate are other prime leak sites as are areas where maintenance was last performed. Due to the small atomic and molecular radii of the elements involved (Table 1), even extremely small holes can yield appreciable leak rates. These atoms are so small that leaks no larger than those caused by slight porosity in welds can cause serious trouble in high-vacuum systems. What Other Types of Leaks are There? Failing to achieve a predetermined and acceptable leak rate, as averaged over a one-hour time period, is not necessarily an indication of a failed leak rate test. You must determine if you are dealing with real leaks or other types of anomalies (e.g., high gas loads, outgassing, virtual leaks, diffusion, permeation, internal leaks or backstreaming). Vacuum system gas load results from sources such as leaks (real and internal), surface conditions (outgassing and virtual leaks) and system materials (diffusion and permeation). The gas load (Fig. 1) is simply the rate gas enters the system volume from areas including: - External and internal leaks
- Outgassing (i.e. gas evolving from surfaces) and/or virtual leaks
- Materials by diffusion and permeation (i.e. gas emanating from or passing through the material in question)
- Process gas flows
| | | Fig. 2. Response of outgassing and real leaks to pumpdown[2] | | At a known temperature, the gas load is the amount of mass (gas) entering the system volume per unit time. Q (gas load, throughput, leak rate) is expressed in units of pressure times volume per unit time such as torr-liters/second, atmosphere-cubic centimeter/second, mbar-liters/sec or Pascal-cubic meter/hour. Each potential leak source can be further broken down. Sources for real leaks (Figs. 2 and 3), which include: - Connections between leak detector and system
- The last component worked on
- Components that have often leaked in the past
- Seals where motion occurs along seal surface
- Sliding seals (e.g. non-bellows valve shaft seals)
- Rotating seals (e.g. fan or fixture drive seals)
- Seals on chamber doors
| | | Fig. 3. Rate of rise due to a real leak[3] | | Sources for outgassing or virtual leaks (Figs. 2 and 4), which include: - Residual solvents following wet cleaning or preventative maintenance
- Liquid leaks (e.g. cooling fluids)
- Trapped volumes of gas or liquid
- Trapped space under non-vented/non-sealed hardware
- Gasses or solvents in spaces with poor conductance
- High vapor pressure materials or products
- Porous materials exposed to liquid or atmosphere
By contrast to other leaks, a virtual leak is a source of gas molecules that are physically trapped within the chamber. As the pressure within the chamber drops during the pumpdown cycle, these molecules are released in a small but steady flow into the vacuum vessel. Water vapor is a good example, which is why one should not leave a vacuum furnace door open any longer than necessary and why one should pump down the furnace as soon as practical. Water vapor trapped inside the furnace will slowly desorb and reabsorb on another area. Since virtual leaks tend to result in spikes of pressure, gauges and leak detectors should be easily able to sense them as non-linear signals. Virtual leaks are most prevalent in four general areas: gaps, cracks, surface contact and trapped (gas) pockets. Good maintenance/repair practices can, in general, avoid their creation. | | | Fig. 4. Rate of rise due to outgassing or a virtual leak[3] | | Note that the slope of the lines shown in Figure 2 represent the pumping speed of the system. Possible sources for internal leaks are: - Process gas delivery valves
- Vent gas valves
- Seals between adjacent internal volumes
- Transfer chamber to process chamber
- Load lock to transfer chamber
- Gas purge or ballast set incorrectly
What is Leak Testing? Leak testing consists of leak checks that should be done on a daily basis to ensure that the vacuum system is maintaining its integrity and leak detection or finding a leak once it is determined that the vacuum system has lost integrity. The vacuum level as indicated by vacuum gauge readings is not always a true indication of the actual conditions within a vacuum furnace. It is possible to have two identical furnaces operating at the same pressure but producing entirely different results from a similar heat treatment. This is due to the relative tightness of each furnace. Most furnaces are equipped with pumping systems sufficient to overcome even reasonably significant leaks. On the furnace with the higher leak rate, air can be continuously infiltrating into the furnace resulting in a higher residual oxygen content than in a tight furnace. The higher oxygen content will adversely affect the heat-treating results. In general, leak testing involves measuring the amount of leakage (vacuum degradation) over time in the vacuum environment. In most shops, a leak-up rate test is performed for a period of one hour, although shorter times may be used (Caution: Use of a 1-, 5- or 10-minute leak-up rate test may give erroneously high leak rate values). Remember, the “leak-up” rate of a vacuum system is a function of actual (real) leaks, internal (through) leaks, outgassing and virtual leaks in the chamber or vacuum system (Table 2).
Why Do I Need to Leak Test?
Everything leaks. And although a leak may be extremely small, it still may pose a problem. Leaks can be inherent in the material, created during the manufacturing process, be introduced during maintenance or repair, or occur over time due to wear, fatigue or stress. The source of a leak may often be revealed by answering the question, “What was the last area worked on or modified?”
However, the question that really matters is this: “Can the system tolerate the leak?” In other words, can the process and equipment survive and be unaffected by the leak? The answer is almost always no. Principles of Leak Testing There are three basic types of flow through leaks in components and vacuum systems (Table 3), which can be defined as follows: - Turbulent flow – Flow in a gross leak situation (10-1 cc/sec and larger). Gross leaks (Table 3) occur, for example, in unsoldered joints, open welds, open valves and plugs left off piping. Typical methods used to find these leaks include soap solution , water immersion, ultrasonic detection and pressure decay.
- Laminar flow – Flow in small or fine leaks (10-1 cc/sec to 10-6 cc/sec). In this case the leak is proportional to the differential pressure (e.g., doubling the pressure across the leak doubles the leak rate). A change in detector gas will, in general, not affect the leak rate. Typical methods used to find these leaks include soap solution, water immersion, ultrasonic detection, pressure decay, halide torch, halogen leak detection and helium mass spectrometry.
- Molecular flow – Flow through extremely fine leaks (10-7 cc/sec or smaller). These occur when the mean free path of the molecule is greater than the diameter of the hole. An increase in pressure differential has no affect on the leak rate. Typical methods used to find these leaks include helium mass spectrometers, radiographic gauges and hydrogen sensitive leak detectors.
Both viscous and molecular flow conditions can exist in a leak situation. Near atmospheric pressure, the flow is viscous, while the lower in pressure one goes, the more the flow conditions become molecular in nature. The type that is dominant will depend on the volume (quantity) of leakage. 1. If the leakage is over 10 -5 torr-liters/second, viscous flow will predominate and the leak rates will be: (a) proportional to the difference between the square of the detector (tracer) gas pressure on opposite ends of the leak. (b) inversely proportional to the detector gas viscosity. 2. If the leakage is under 10 -7 torr-liters/second, molecular flow will predominate and the leak rates will be : a) proportional to the difference in pressure across the leak. b) inversely proportional to the square root of the molecular weight of the gas. 3. If the leakage is between 10 -5 and 10 -7 torr-liters/second, flow conditions will be in the transition region and the effect on leak rate is difficult to predict. The most influential changes to the system pressure depend on: - The maximum detector gas partial pressure (after the leak is covered by the detector gas)
- The time required for the detector gas to reach its maximum partial pressure
- The time required for the partial pressure of the detector gas to drop from maximum to minimum
- The rate of increase of the detector gas partial pressure in the vacuum system
How to Ensure the Most Accurate Leak Test Value For best results, leak testing should be done in a clean, cold, empty and outgassed chamber. To accomplish this once a vacuum furnace has been put into service, a furnace burnout cycle should be conducted. The purpose of a burnout is to condition the hot zone. It is important to note, however, that a burnout cycle should never be conducted in a badly leaking vacuum furnace. If oxygen is present during a burnout cycle, in the case of metallic shields, these can be damaged or destroyed by oxidation (i.e. loss of the ability to re-radiate). If the hot zone is graphite construction, the graphite can be severely oxidized, compromising thermal integrity. In either hot zone construction, there is a danger of metallizing the heating-element electrical insulators, which can result in short-circuits or heating-element failures. So, before a burnout cycle is run, pump on the empty furnace for approximately four hours, and if the vacuum level fails to lower to acceptable limits, a leak check to find and seal gross leaks needs to be done. A furnace burnout typically involves heating the furnace to a temperature (approximately) 100°F (38°C) above the maximum process temperature (but less than the rated maximum operating temperature of the furnace) followed by a vacuum cool. If you are running a standard furnace at temperatures below 2000°F (1090°C), the burnout temperature should still be in the 2000-2300ºF (1090-1260°C) range. Fixtures, baskets, parts and work thermocouples are removed from the furnace. A partial pressure gas (nitrogen, argon, helium or hydrogen) is normally used to aid in the cleanup cycle. A typical set of burnout instructions might be as follows (for a standard graphite or metal-lined hot zone): 1. Ramp the furnace at 20ºF (10°C) to a setpoint of 100ºF (38°C) over the maximum normal daily cycle operating temperatures. Do not exceed the recommended maximum operating temperature of the furnace. 2. During the ramp portion of the cycle, run in a partial pressure of 1000-2000 microns (1-2 torr) typically with either nitrogen or argon. If a hydrogen partial pressure is used, appropriate safety considerations must be taken and the furnace temperature monitored very carefully to prevent a runaway condition due to the high heat-transfer characteristics of hydrogen. 3. Once the furnace reaches burnout temperature, the furnace should be allowed to remain in this condition for approximately two hours. At the end of the two-hour soak, the partial-pressure event is stopped and the furnace allowed to pump to a harder vacuum level. 4. The furnace is then soaked at the burnout temperature for an additional hour in the hard vacuum condition. 5. The heat is turned off and the furnace is allowed to vacuum cool to between 500-800ºF (260-425°C) so that vaporization can take place of previously heated materials. Note: To obtain the best possible leak rate, the furnace must be allowed to vacuum cool to below 120º F (50°C). 6. Backfill and cool to room temperature, open door, inspect hot zone and elements, check element to ground resistance per manufacturer’s specifications. 7. When finished, the furnace can be leak tested and then placed back into production. The leak rate test is normally performed immediately after the bakeout cycle without opening the furnace to atmosphere. Pump the system down to normal vacuum operating pressure (or less) then isolate the pumping system from the furnace chamber. The vacuum level is recorded after 30 minutes and again after 60 minutes. The leak rate can then be calculated in microns per hour and compared to previous values or the original equipment manufacturer’s requirements. Vacuum furnaces should not have leak rates exceeding 10 microns per hour at a pressure of 100 microns or less. For older furnaces, leak rates of 20 to 25 microns per hour are not unusual. For vacuum oil-quench furnaces, the quench tanks will typically have leak rates of 100 microns or less. These leak rates ensure that the volume of impurities that may be leaking into the furnace is sufficiently low so as not to cause any significant detrimental effects to the materials being processed. A furnace exhibiting a leak rate greater than these limits should not be used for production until repair of the leak. In this case, the normal procedure is to backfill the furnace with nitrogen, but do not open the chamber to atmosphere. All thermocouple fittings and other vacuum feed-thrus should be tightened. The furnace can then be re-tested for leak rate as before. Failure of the second leak rate test is an indication that the furnace requires more extensive maintenance, possibly including helium leak checking. Leak Testing Explained There are many methods used to measure leak rate (Table 4) and various types of detector gases or leak-detecting (tracing) agents (Table 5). The choice of tracing agent usually depends on the measurement method used. Selection of the best method for a specific application requires consideration of economics, accuracy, tolerance to environmental conditions, leak-rate requirements and equipment limitations. In most heat-treat applications, helium is the preferred detector gas. To perform a leak-up rate check, pump the furnace down to ultimate pressure with the heat turned off and the furnace cold (ambient temperature or below). Record the vacuum level and the time. Next, isolate the furnace from the pumping system by closing the vacuum valve(s) to the chamber. Allow at least one hour to obtain an accurate leak-up rate. (Note: This step is often shortened to only a few minutes, but this is poor practice and should be avoided.) Record the time and vacuum level. The leak-up rate is the difference in the vacuum levels divided by the elapsed time and is expressed in microns/hour (mbar/h). Test Methods | | | Fig. 5. Portable helium mass spectrometer[2] | | There are three general categories of leak detection procedures: - Effect-of-leak types – pressure decay (differential, increase), vacuum decay
- Amount-of-leak types – mass flow (inside/out, outside/in, accumulation), carrier gas, residual gas analysis (RGA)
- Traditional types – immersion, sniffing
The most common procedures for detecting leaks in vacuum furnaces are the solvent (alcohols and acetone) and mass spectrometer procedures. Total Pressure Measurement
It is often possible to use the existing vacuum gauges on a system as a basic leak detection system. However, the change in gauge indication is subject to a number of conditions that can cause the gauge to either increase or decrease in its value. These conditions include: the change in leak rate when the leak is covered by the detector gas; the change in gauge response to the detector gas (compared to its response to air); and the difference in pumping speed of the detector gas (compared to the speed for air). For thermal conductivity gauges, the most effective technique is to use two detector gases: one that causes an increase in gauge response and one that causes a decrease. The procedure used is to alternate the gases and measure the degree of gauge response. This difference can be as high as a factor of five. Pirani gauges are preferred over thermocouple type for this type of leak detection. For ionization gauges, the technique is similar, with hot cathode gauges preferred over cathode models. SolventsThe use of a solvent test method is simple but effective for locating gross leaks; that is, leaks considered in the intermediate and large size ranges. If a thermocouple gauge is connected on the pump side of the system such that it reads manifold pressure and the system can be evacuated to a range of at least 200 microns (0.2 torr), then a solvent such as acetone (preferred) or alcohol can be carefully sprayed on a suspect area and any change in vacuum level inside the chamber, based on the vacuum gauge reading, noted. In using this method one must be careful to allow enough time (up to 20 seconds) for a pressure increase to occur. This procedure is more sensitive at lower pressures. Solvent checking is typically used to enable the system to be evacuated into the range where a mass spectrometer instrument can be used to check for smaller leaks. Be sure to observe all required safety precautions when using hazardous solvents, including proper ventilation and spill containment. Remember also that these solvents will remove paint! If a leak is located, a temporary sealant such as Glyptal red alkyd lacquer, Kinseal clear vacuum sealant, vacuum seal putty or wax can be used to patch the area and allow leak checking to continue. A common mistake is to neglect to permanently fix the problem after testing is completed. Residual Gas AnalyzersA residual gas analyzer (RGA) can be used to measure gross and fine leaks in the partial pressures of a range of gases present in a vacuum system. RGA’s can also be used to locate leaks by applying any gas to small areas on the outside of the vessel and to look for an increase in partial pressure of that particular gas species. The trend mode is typically set to helium (mass 4) and the unit differentially pumped for high system pressure. The RGA cannot measure the leak in a quantitative way since it is very difficult to determine the speed of the leak. Helium Mass SpectrometersA helium mass spectrometer (Fig. 5) is a highly accurate instrument for locating very small leaks or leaks in hard-to-reach areas. In some instances, it is necessary to “bag” or isolate a specific area on the furnace and inject helium into the contained space. This dynamic, nondestructive technique is sensitive enough to check parts that have moving seals or those that may leak only during the transition from pressure to vacuum (or vice versa). A mass spectrometer can detect extremely small amounts of helium (or another tracer gas). When the gas enters the spectrometer tube, it is ionized and accelerated. These high-speed charged particles are then exposed to a magnetic field perpendicular to their direction of motion. What results is a force perpendicular to both the velocity vector and magnetic field. This force causes the particles to follow a curved path, the radius of which depends on the mass of the particle, allowing separation of the particle stream into different ions. A properly positioned collector plate (ion detector) enables the concentration of any gas to be very accurately measured. Every electron given up by the collector plate equates to the presence of one helium ion. The amount of helium collected is then converted to a leak rate. Helium is the tracer gas of choice because it is inert, nontoxic, relatively inexpensive (in small quantities) and not easily absorbed. Helium also easily flows through small leaks and has only a trace presence in air (usually 5 ppm). Mass Spectrometer Leak Testing Tips | | | Fig. 5. Automated leak check software[3] | | Vacuum pumps are very efficient at pumping large atoms and molecules (water and hydrocarbon vapors, oxygen and carbon dioxide, for example). However, they are inefficient at pumping helium. This allows a greater amount of helium to reach the mass spectrometer for measurement. Mass spectrometer leak testing requires that the unit be exposed to helium leaking for only 3 to 4 seconds. However, as with the solvent test method, a dwell or lag time between test areas is needed to prevent false readings. So being too quick (aggressive) in finding leaks using helium is a bad practice. This is especially true since in most applications helium is squirted on the outside of suspected areas with a small blast of helium from a pressurized tank in such a way that the helium will pass through the leak and into the vacuum system and be intercepted by the leak detector. There is a tendency to want to “move on” before allowing enough time for the helium to migrate from outside to the detector. In utilizing helium, checking should always begin at the bottom of a pressurized system and at the top of an evacuated system. When dealing with moving or transition seals (e.g., vacuum seals that must also withstand pressurization), it may be necessary to “bag” the area under investigation, using a plastic bag and tape to seal off a component, and then inject helium into the closed area. Remember that you are dealing with total leakage within the enclosed space. Many modern vacuum furnaces come equipped with software (Fig. 6) to make the job of leak detection easier.
Backfill Lines...and More
There is no acceptable minimum leak size. Since the vacuum chamber is not capable of determining how many leaks are present or their sizes, the effect is cumulative. For example, five small leaks can be more harmful than one large leak if the sum of their leak rates exceeds that of the large leak.
When leak testing a vacuum furnace, do not forget to check the gas backfill lines (including all fittings) from the gas supply to the furnace. It’s good practice to install vacuum-tight shutoff valves near the source (just inside the building if using a cryogenic system located outside) and at the equipment. These lines are often pressurized and “soap tested” when first installed. However, that does not guarantee that they do not leak. The gas inside backfill lines often travels at near supersonic velocities, and a pinhole leak in a line will draw in air via a venturi action. Always perform leak-up tests first with the backfill lines closed and then with them open (up to the shutoff valve near the source) to confirm that the lines are not leaking. The backfill lines may then be evacuated using the vacuum furnace pumping system up to the source and the lines themselves vacuum checked utilizing a helium leak detector.
In addition, clean and accurate vacuum measuring devices are essential for obtaining a meaningful value of leak-up rate. The need for periodic checks and annual calibrations of all vacuum instruments against known standards cannot be overemphasized.
Finally, there are differences between North American and European leak-up rate specifications. In North America, the specs usually involve a fixed leak-up rate and do not take into account the chamber volume. OEM specifications for new equipment vary from 2 to 10 microns/hour (0.003 to 0.013 mbar/h) In Europe, the leak-up rate factors in chamber volume and is expressed in units such as mbar-L/s (millibar-liter per second).
Final Thoughts
A comprehensive preventive maintenance program is essential to minimizing downtime due to vacuum leaks. Proper care of pumps, replacement of “O” rings as they age, cleaning of flange sealing surfaces and regular inspection of vacuum feedthrus will help prevent leaks. In addition, daily leak checks or continuous monitoring of vacuum levels during processing can also help to identify potential problems before they develop into major repairs.
Next Time: Part 13 of this series discusses the importance of cleaning and discusses/compares solvent-based systems to aqueous solutions as well as talks about alternative cleaning methods that have been used over the years.
Daniel H. Herring
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.
References
1. Practical Vacuum Systems Design, The Boeing Company.
2. Leak Detection, Applications & Techniques, Varian Vacuum Technologies.
3. Grann, James, "Understanding the Difference Between Linear and Non-Linear Leak Rates, Symposium on Vacuum Furnace Maintenance," ASM International, October 2007.
4. Herring, D. H., "The Why, When and How of Leak Checking a Vacuum Furnace," Heat Treating Progress, September/October 2003.
5. The Nature of Vacuum, SECO/WARWICK Corporation.
6. Brunner, William F., "Vacuum Leak Detection," American Vacuum Society, 1981. |
|
|
Written by Gus
|
|
Friday, 13 August 2010 10:25 |
Nadcap From Wikipedia, the free encyclopedia Nadcap (formerly NADCAP, the National Aerospace and Defense Contractors Accreditation Program) is a global cooperative standards-setting program for aerospace engineering, defense and related industries. // [edit] History of Nadcap Nadcap program as a part of PRI (Performance Review Institute) was created in 1990 by the Society of Automotive Engineers and is headquartered in Warrendale, Pennsylvania. Nadcap's membership of "prime contractors" convene to coordinate industry-wide standards for special processes and products. Through the Performance Review Institute, Nadcap provides independent certification of manufacturing processes for the industry. PRI's mission is to "provide international, unbiased, independent manufacturing process and product assessments and certification services for the purpose of adding value, reducing total cost, and facilitating relationships between primes and suppliers." Branch offices of Nadcap are located in London, Beijing, and Nagoya. [edit] Fields of Nadcap activities Nadcap program give accreditation for such special processes in aerospace and military industry as: [edit] The Nadcap program and industry PRI schedules an audit and hires an approved auditor who will conduct the audit against an industry agreed standard using an industry agreed checklist. At the end of the audit, any non-conformity issues will be raised and non-conformance reports issued. PRI will administer close out of the non-conformance reports and upon completion will present the completed audit pack to a 'special process' task group made up from members of industry who will review it and vote on its acceptability for approval. The Nadcap Prime Contractors are: [edit] Nadcap Meetings Nadcap meetings are held several times a year in different locations worldwide. For example, the 2007 meetings were held in Redondo Beach (California) in January; Paris (France) in April; Istanbul (Turkey) in July. The October meeting was held in Pittsburgh (Pennsylvania). Other locations can be in Europe or Asia. In February 2008 the meeting took place in Rome (Italy). The July meeting is being held in regular place in Pittsburgh. During these meetings there are Task Group discussions and workshops (with participation of primes, suppliers, and PRI staff). [edit] Nadcap Training During the periodic meetings, Nadcap performs training for airspace industry suppliers in different fields, such as: - Pyrometry
- Root Cause Corrective Action training - RCCA
- Special processes, such as coating, NDT, chemical processing, etc.
- Internal audits
- AS/EN/JISQ 9100
- Problem Solving Tools
The Nadcap training takes place in different regions — USA, Europe, Far East, simultaneously there are meetings of Task Groups (with participation of primes and suppliers). At times these trainings are offered at times separate from the full Nadcap meetings. [edit] References |
|
Last Updated ( Monday, 16 August 2010 13:29 )
|
|
Written by Administrator
|
|
Thursday, 23 October 2008 16:46 |
Metal fatigue is caused by repeated cycling of of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe. The process of fatigue consists of three stages: |  Schematic of S-N Curve, showing increase in fatigue life with decreasing stresses. |
Stress Ratio The most commonly used stress ratio is R, the ratio of the minimum stress to the maximum stress (Smin/Smax). -
If the stresses are fully reversed, then R = -1. -
If the stresses are partially reversed, R = a negative number less than 1. -
If the stress is cycled between a maximum stress and no load, R = zero. -
If the stress is cycled between two tensile stresses, R = a positive number less than 1.
Variations in the stress ratios can significantly affect fatigue life. The presence of a mean stress component has a substantial effect on fatigue failure. When a tensile mean stress is added to the alternating stresses, a component will fail at lower alternating stress than it does under a fully reversed stress.   Preventing Fatigue Failure The most effective method of improving fatigue performance is improvements in design: -
Eliminate or reduce stress raisers by streamlining the part -
Avoid sharp surface tears resulting from punching, stamping, shearing, or other processes -
Prevent the development of surface discontinuities during processing. -
Reduce or eliminate tensile residual stresses caused by manufacturing. -
Improve the details of fabrication and fastening procedures
Fatigue Failure Analysis Metal fatigue is a significant problem because it can occur due to repeated loads below the static yield strength. This can result in an unexpected and catastrophic failure in use. Because most engineering materials contain discontinuities most metal fatigue cracks initiate from discontinuities in highly stressed regions of the component. The failure may be due the discontinuity, design, improper maintenance or other causes. A failure analysis can determine the cause of the failure. |
|
Last Updated ( Thursday, 23 October 2008 17:11 )
|
|
|
Stress Corrosion Cracking |
 |
 |
 |
|
Written by Administrator
|
|
Thursday, 23 October 2008 00:00 |
|
Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material, and tensile stress. Temperature is a significant environmental factor affecting cracking. For stress corrosion cracking to occur all three conditions must be met simultaneously. The component needs to be in a particular crack promoting environment, the component must be made of a susceptible material, and there must be tensile stresses above some minimum threshold value. An externally applied load is not required as the tensile stresses may be due to residual stresses in the material. The threshold stresses are commonly below the yield stress of the material. Stress Corrosion Cracking Failures Stress corrosion cracking is an insidious type of failure as it can occur without an externally applied load or at loads significantly below yield stress. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component. Pitting is commonly associated with stress corrosion cracking phenomena. |  
| Aluminum and stainless steel are well known for stress corrosion cracking problems. However, all metals are susceptible to stress corrosion cracking in the right environment. Controlling Stress Corrosion Cracking There are several methods to prevent stress corrosion cracking. One common method is proper selection of the appropriate material. A second method is to remove the chemical species that promotes cracking. Another method is to change the manufacturing process or design to reduce the tensile stresses. AMC can provide engineering expertise to prevent or reduce the likelihood of stress corrosion cracking in your components. |
|
Last Updated ( Thursday, 23 October 2008 16:23 )
|
|
Heat Treatment Terminology |
|
|
|
|
Written by Administrator
|
|
Thursday, 23 October 2008 00:00 |
|
Below are some common heat treating terminology as used by individuals in the steel industry. These terms are not being used in a specification and no specific temperatures are identified. Aging: Describes a time–temperature-dependent change in the properties of certain alloys. Except for strain aging and age softening, it is the result of precipitation from a solid solution of one or more compounds whose solubility decreases with decreasing temperature. For each alloy susceptible to aging, there is a unique range of time–temperature combinations to which it will respond. Annealing: A term denoting a treatment, consisting of heating to and holding at a suitable temperature followed by cooling at a suitable rate, used primarily to soften but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be, but is not confined to, improvement of machinability; facilitation of cold working; improvement of mechanical or electrical properties; or increase in stability of dimensions. The time–temperature cycles used vary widely both in maximum temperature attained and in cooling rate employed, depending on the composition of the material, its condition, and the results desired. |
|
Last Updated ( Thursday, 23 October 2008 16:19 )
|
|
Read more... [Heat Treatment Terminology]
|
|
|
|
|
|
|
Page 1 of 3 |
|
Heat Treatment Articles
