Both normalized SAE-AISI and SAE-AISI M steel are iron alloys. There are 10 material properties with values for both materials. Properties with values. M is a low alloy, vacuum melted, steel of very high strength and toughness. It is a modified steel with silicon, vanadium and slightly greater carbon and. M high strength low alloy steel. M (M) is a through hardened low- alloyed with very high strength. It is a modified AISI with silicon, vanadium.
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Many landing gear, flap track, flap carriage, and other flap actuating components on Boeing airplanes are made of high-strength alloy steels, such as M, Hy-Tuf, M, and M. These components provide structural benefits e. Other steels in use, including 9Ni-4Co High-strength alloy steels referenced in this article generally have been heat-treated above ksi [, psi]; most have been heat-treated above ksi.
Airline personnel should follow proper maintenance procedures and Boeing-provided rework practices, checklists, and planning guidelines during maintenance and overhaul of these components. This will help operators achieve the benefits associated with high-strength alloy steels and avoid potential safety issues resulting from damage caused by stress concentrations, detrimental surface conditions, corrosion, improper processing, or other factors.
This article discusses some factors that cause damage in service or during overhaul. Most can be attributed to a lack of familiarity with high-strength alloy steels.
Operators usually recognize the benefits of using these steels; however, certain characteristics of the steels are not always given proper consideration during component maintenance or overhaul. These characteristics, including sensitivity to corrosion pitting, susceptibility to microstructural damage resulting from embrittlement, and notch sensitivity, can lead to rapid crack growth in some load environments. Components made of high-strength alloy steel generally weigh less and require less space to house than components made of lower strength alloys.
Using high-strength alloy steel for component design provides an opportunity to do the same job with less material. When properly maintained and overhauled, high-strength alloy steel components demonstrate high levels of service reliability. The decision to use high-strength alloy steels is based on weight and economic factors. Airframe space for gear components may be reduced because of smaller diameter shock strut components, smaller pins reduced space for jointssmaller diameter trucks and axles, and, in some instances, smaller drag brace, side brace, and attach fittings.
By reducing the space required for these components, the wheel well size can be minimized and aerodynamic surfaces can be optimized, which allow an increase in fuel tank size optimal wing spar location or additional space for other uses.
The use of high-strength alloy steel parts is economical because it reduces weight, thereby allowing for more efficient aerodynamic surfaces and providing the potential for increased payload and fuel. For example, the trailing edge of the wing is relatively shallow. Using high-strength alloy steel flap tracks, flap carriages, and flap actuating components reduces the profile and decreases spatial envelope requirements while meeting or improving aerodynamic requirements.
This also optimizes wing shape and reduces the potential need for bulging aerodynamic surfaces, which in turn reduces drag and increases airplane performance. Following proper rework practices and using Boeing-provided documents during maintenance and overhaul are necessary to achieve the benefits associated with high-strength alloy steel components and help ensure safe airplane operation. Airline personnel who participate in component rework, maintenance, and overhaul tasks should be familiar with the properties of high-strength steels and understand the negative effects that can result from.
Improper rework practices can result in unscheduled maintenance or surface damage that causes crack initiation. Maintenance efforts focus on corrosion prevention and removal in addition to normal checks for wear and free play. High-strength alloy steels can experience rapid crack propagation from stress corrosion under certain loading conditions. Therefore, surface damage detection is important during overhaul and on components in service. Removing visible surface corrosion before pitting begins such as during a C-check helps prevent conditions that can lead to crack initiation.
The best safeguard against corrosion is to ensure that finishes conform to the design and that design improvements are incorporated as minor changes whenever possible.
Components manufactured from steel alloys heat-treated above ksipsi should be reworked in accordance with guidelines in Component Maintenance Manuals CMM, and Although these guidelines apply directly to landing gear components, they can be used to plan the overhaul rework of all high-strength steel components.
In addition, airline personnel need to understand the importance of maintaining component finishes while in service in situor stee, the airplane.
This includes repairing damaged finishes to prevent corrosion and ensuring that solvents and materials that come in contact with the finishes do not result in premature degradation and unscheduled component removal.
Boeing documentation describes the methods for detecting base metal damage while in service and during overhaul. Common techniques include detailed visual inspections and other nondestructive 4340n methods, such as magnetic particle inspection MPI and fluorescent penetrant inspection FPI.
See SOPMs and Ultrasonic or eddy current inspections also may be useful for in situ inspections.
Boeing also is developing supplemental, specialized techniques, such as the Barkhausen inspection, to detect base metal heat damage under chrome plating or other protective finishes. This technique can be used successfully to screen components with suspect damage. For example, if an axle fractures as a result of chrome-grinding heat damage during manufacture or overhaul, the Barkhausen inspection allows other suspect components to be screened without first performing a chrome strip and temper etch e.
This section provides guidelines for reworking high-strength alloy steel components and describes some of the implications of improper rework procedures. Special attention is given to protective finish runouts adjacent to stress concentration details. Any rework or repair must not increase stress concentrations that degrade component durability. When a surface is 4430m or ground to remove damage, the reworked area should be shot-peened with proper overlap onto the existing shot-peened surface.
For example, when a coating such as stee or nickel plating is applied to surfaces to prevent wear or corrosion, the coating must exhibit proper runouts that terminate before the tangent of fillet radii, edges, or other shape changes. Boeing SOPM guidelines should be followed for the rework of any component and for all types of plating or coating.
Rework or overhaul of components should not introduce stress concentrations, or otherwise increase stresses, which can reduce the service life of a component below that of the original design configuration. Stress concentrations can stdel to initiation of cracking by fatigue, stress corrosion, or hydrogen-assisted stress corrosion. These cracks may result in a fracture or scrap of a component when found while in service or during overhaul.
The following are examples of stress concentrations that can lead to cracking. Transitions or radii that are sharper than original design.
When removing damaged material from part surfaces during rework, the new transitions or radii should not cause an unacceptable increase in stress concentration at the location or degrade the stesl design features.
When locally machining out corrosion or damage during 4340mm, a gradual transition into the reworked depression is necessary. The intent is to remove the least amount of material possible while ensuring that all discrepant material is removed and the original design strength and durability are maintained. There are few 4340mm to restore these machined depressions to meet interface requirements.
300M Alloy Steel (4340M)
One type stee, rework or overhaul, sulfamate-nickel plating, is common on shock strut cylinder diameters and is used to repair lug faces to design dimensions as follows: Abrupt changes in sections, holes, and sharp-cornered keyways should be avoided. Proper design will reflect generous fillets, gradual changes of shape, stdel the use of relief grooves in areas of high stress.
Finer surface finishes also may be needed to eliminate unnecessary stress concentrations, especially in areas of machined radii or undercuts. Overhaul should reflect the same careful, detailed review that occurred during the original design. Plating conditions and runout controls that are not in accordance with design standards.
During overhaul, many landing gear components are completely stripped to replace nickel and chrome plating. In most instances, these repairs involve rework of the base metal. The new plating deposits frequently are thicker than the original design configuration steel deterioration of the plating adhesion. Through-thickness cracking also can lead to fatigue or stress corrosion cracking of the base metal beneath the plating. Visual evidence of chicken-wire cracking after chrome grinding indicates poor chrome quality and also may indicate the possibility of base metal heat damage.
4340M / 300M VAR Cr-Ni-Mo-V Aircraft Quality Alloy Steel
Chicken-wire cracking noted in SOPM indicates that the chrome should be stripped and replated. If the plating runouts are blended or machined to remove the abrupt plating edge, the techniques must be well controlled 44340m avoid damaging the adjacent base metal. Improper blending can remove the required shot-peened layer or create undercuts or grooves at the edge of the plating that can cause cracking in service.
Several in-service fractures have been attributed to improper plating technique, poor-quality plating, improper runout conditions, and 3440m metal damage caused by poor blending or machining control. Proper use of special plating techniques, such as conforming anodes and robbers, can control plating thicknesses and runouts.
Aero 22 – Maintenance of High-Stregth Alloy Steel Components
This can reduce the possibility of chrome chicken-wire cracking and poor runout details. Plating into a transition radius transition or undercut will create a stress concentration that can cause crack initiation. For example, figure 1 shows an outer cylinder clevis plated into the lug transition. In service, fatigue cracking initiated at the plating runout led to lug fracture.
Corrosion pits are stress concentrations. As the pit forms, it damages the shot-peened layer locally at the surface. The pit then grows through the compressive layer, and the change in residual stress state and the pit geometry initiate stress corrosion cracking.
This type of cracking most often occurs on surfaces that are both prone to corrosion and exposed to sustained tensile stresses while in service, such as the lower surface of landing gear trucks, axles, and the surfaces of forward and aft trunnions. Corrosion pitting also can lead to fatigue crack initiation depending on the component, the location of pitting, and cyclic loading conditions.
In these cases, the cracks can propagate to the critical length and result in ductile fracture of the component. The degree of cracking tolerated before fracture varies by component, crack location, and component loading conditions. To prevent excessive corrosion, thorough visual inspections should be performed on a regular basis steep evaluate the condition of the protective finishes.
Damage should 430m repaired soon after it is found. Touching up damage to accessible enamel and primer in a timely manner can prevent the formation of corrosion pits and reduce the need for excessive rework during overhaul.
Rework that requires low-hydrogen embrittlement LHE cadmium stylus plating should be performed when the component is not loaded. When the component is removed for overhaul, all evidence of corrosion must be removed and stefl restored to design requirements or better.
M (Modified) Steel | Rickard Metals
The sequence of rework operations is provided in CMMs, and Landing gear truck fractures have occurred in service because of corrosion on the inner diameter of the main gear truck beam figs. These fractures may be caused by a combination of degraded protective finishes on seel truck inner diameter, poor drainage, and steek with the corrosive chemicals in washing solutions or deicing compounds.
Truck fractures most often occur at maximum ground loads such as after fueling or during preflight taxi. Figures 4 and 5 show a drag brace from which corrosion was not removed completely during overhaul.
The part was subsequently shot-peened, and new protective finishes were stewl over the residual active corrosion. Sreel resulted in crack initiation and propagation while in service and the eventual fracture stwel the component. Stress concentrations can be created by mechanical damage that compromises the protective finishes and alters the compressive shot-peen layer.
This damage often is caused by improper maintenance practices such as jacking adjacent to a jack pad or an inadvertent impact with tools or ground-support equipment e. Although high-strength alloy steels are hard and resist dents, scratches, and nicks, stress concentrations caused by mechanical damage can dramatically reduce the service life of a component. High-strength alloy steel components also can be damaged by mishandling during shop rework e.
Possible mechanical damage to a high-strength alloy steel component should be evaluated by the operator and repaired as needed.
If the damage is local and widespread deformations are not evident, repair may be similar to that required for corrosion and pitting.