Aeronautical requirements for hardening
Written in the framework of the "Fluids and quenching systems" committee
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
1. Reference systems and standards dedicated to aeronautics
As in the automotive industry, and as for any ISO 9XXX type quality assurance system (ISO 9100 for aeronautics), the first and most important requirement is to meet the requirements of the designer of the part to be processed (also called "prime").
The requirements included in the technical definition of the article ("the plan") take precedence over any other requirement, even normative. For heat treatment, the drawings refer to international standards (ISO, AMS,...) or to the designer's standards (BAC, Pr, DMP, PCS...). The latter are sometimes based on international standards and add additional requirements.
Credits: SAFRAN, The BOEING Company, SAE international
Compliance with the requirements is the responsibility of the designer. For this, he relies on the following approaches:
Qualification is a prerequisite for processing aeronautical parts. This process allows the designer to verify that the equipment, procedures, and skills allow compliance with all of his requirements
Some companies add, after the qualification, an industrial validation process. In this case, the aim is to demonstrate that the proposed production parameters, from the furnace setting to the controls, including the constitution of the charge, the washing, etc., allow the production of parts in conformity with their technical definition and in a repeatable and reproducible way. Please note that many parameters, practices and procedures must be approved by the designer before being implemented.
In order to monitor its suppliers, the designer can rely on the NADCAP (National Aerospace and Defense Contractors Accreditation Program) accreditation program, run by the PRI (Performance Review Institute).
Accreditations are issued following audits based on questionnaires (checklists) that verify compliance with customer and designer requirements. Some good practices, shared by all aircraft manufacturers, are also added to these questionnaires. They then become de facto additional requirements.
NADCAP accreditation does not replace the qualification and validation of the designer. However, it does allow for the selection of suppliers who meet most of the requirements applicable to the aerospace industry.
It is of course impossible to review in this guide all the requirements of all aeronautical designers. We will focus on the requirements for hardening systems defined in AMS2759 and AMS2769 for steels, AMS 2770 (*) for aluminum, AMS2801 for titanium, AMS 2774 for nickel-based alloys and associated NADCAP questionnaires. These standards are the basis for many aerospace specifications around the world.
(*): The processing of aluminum alloy castings is governed by AMS2771 and the processing of raw materials by AMS2772. But the differences between these 3 standards are marginal for the subject of this guide.
This chapter is based on the indices of these standards that are in effect at the time this guide is published. It represents the state of the specifications. Where they do not contradict customer requirements, the application of the good practices set out elsewhere in this guide is strongly recommended.
2. quenching system requirements
2.1 Equipment requirements
Regardless of the material processed, steel, aluminum alloy or titanium, AMS requires :
A quenching tank of a sufficient size to allow
o the complete immersion of the parts
o the free circulation of the quenching fluid around the parts.
o to respect the maximum temperature rise allowed.A system allowing to absorb the calories brought by the parts to be soaked (via a cooling system and/or sufficient volume and/or the effect of the agitation).
An agitation or fluid circulation system. Agitation by air injection is prohibited by AMS 2770 for aluminum alloys. In some cases, agitation of the parts is allowed as the only means of agitation (thin aluminum parts for example).
The temperature of the quenching fluid must be measured, controlled (regulated) and recorded.
In addition, for vacuum processing, AMS 2769 specifies that the transfer lock and the oil quenching cell must minimize the pollution of the processing laboratory by oil vapors.
In the case of quenching a steel in a polymer solution, a concentration measurement must be performed before production. This measurement can be done by an automatic means.
Since transfer times are very short for aluminum alloys, the tank should be as close as possible to the solution furnace.
2.2 Fluid temperature requirements
Temperature ranges (e.g. for oil quenching) or a maximum bath temperature before quenching are specified for each type of material and quenching fluid. Maximum temperature peaks after quenching are also defined.
For steels, the AMS 2759 (steel processing) gives as default temperature of the quenching oil 16 to 71°C, with a maximum allowed peak at 93°C. Remember, the designer of the part to be treated may request a different temperature. Any non-compliance with these temperatures must be subject to prior agreement from the designer.
These temperature requirements, either from the AMS or from a specification from the designer of the article to be treated, are therefore an important input for the choice of the quenching oil to be used.
AMS 2770 requires water at a maximum of 32°C for water quenching of aluminum alloys before quenching. A temperature increase of 14°C maximum without exceeding 38°C. Higher temperatures are required for quenching parts from forged products.
For the use of polymers, temperature guidelines are usually given by the designer. Default polymer concentrations are also given by Table 4 of AMS 2770 for aluminum.
AMS 2801 (titanium alloy processing) recommends, unless otherwise specified by the designer, water below 38°C at any time during the quench, or a polymer bath below 49°C. In the case of oil quenching, the temperature criteria are the same as for AMS 2759.
Remember, these are default guidelines that apply if the designer has no specific requirements. Otherwise, the designer's requirements must be followed. The designer may also accept other practices (acceptance must always precede implementation).
2.3 Pyrometric requirements
For the aeronautical sector, the standard governing pyrometry is AMS2750. At the date of publication of this guide, it is at index E.
This standard, which is widely used and required, deals mainly with the following topics
The choice, the use and the calibration of the sensors (Thermocouples for example), for the oven and for the realization of the tests and verification.
Instrumentation, devices for the operation of thermal treatment plants as well as for their monitoring.
The definition of the pyrometric chains required for a given equipment (oven) and the level of monitoring that follows.
The verification of the accuracy of the measurement chains (SAT - System Accuracy Test)
Temperature Uniformity Survey (TUS)
This standard is very complete. It is intended to cover all types of ovens, for all uses.
It can be concluded that the level of monitoring (type and frequency of controls) is inversely proportional to the confidence one can have in the measurement of the temperature of a furnace.
The more a furnace is equipped with independent and redundant measurement chains, with robust and precise sensors (noble metals for example), and benefits from an adapted and efficient preventive maintenance, the more the monitoring will be spaced out, and thus less expensive.
Conversely, an oven equipped with a single thermocouple must be checked more frequently.
2.4. Pyrometric requirements for quenching
If the designer's requirement imposes a temperature of the quenching liquid (minimum, maximum or both), then the quenching tank must be equipped with a temperature control chain. This temperature must be recorded (AMS 2750E §3.3.3.2).
The calibration of the instrumentation of this control chain must be done semi-annually. An exception, for the case of a recording made via a channel of a recorder of the furnace, the frequency of calibration of this channel of the recorder is that applied for all other channels of the recorder (AMS 2750E Table 3 Note 5).
A System Accuracy Test (SAT) is required every 6 months (AMS 2750E Tables 6 and 7). The maximum allowable temperature deviation for this test is +/-2.8°C (with a maximum allowable offset of +/-6°C).
A Temperature Uniformity Survey (TUS) is not required for a quenching tank.
3. Agitation of the quenching fluid
Agitation is required for all material families (no dead bath). Agitation of the fluid by compressed air is prohibited for aluminum alloys.
The only agitation of the parts can be accepted in certain cases (fine parts in aluminium or titanium alloys for example).
Verification of the proper operation of the liquid agitation system is required prior to treatment. The preventive maintenance plan for the quenching system should include actions to ensure its proper operation and efficiency.
4. Quenching time
The immersion time of the load in the quenching fluid must be sufficient to allow the parts to reach the temperature of the quenching fluid.
Unless specifically required by the designer, minimum quenching times are given for quenching aluminum alloys. They are in the order of 1 to 2 minutes after the end of boiling water or 1 minute per 25 mm (1 inch) of cross-section, whichever is longer.
For steels and titanium alloys, there is no guideline or chart for this step. This time depends on the section of the treated parts, the constitution of the load, the design of the quenching system, ... it must be defined on a case by case basis, verified, and fixed in the data sheet of the quenching operation.
Please note that the material specification or heat treatment may require a certain temperature to be reached before tempering, e.g. room temperature.
If the quenching and washing baths are above this required temperature, provisions should be made to ensure cooling of the parts before tempering.
When a delay between quenching and tempering is specified, the starting point of the delay is the time of immersion of the parts, not the time of their removal from the bath.
5. The level of the quenching tank
There is no special requirement for the level or monitoring of the liquid level in a quenching tank.
We find requests such as "the tank must allow for complete immersion of the parts". This is common sense from both a metallurgical and safety point of view.
6. Choice and monitoring of quenching fluids
Whatever the system and the quenching fluid used, they must allow a "homogeneous" cooling of the parts to be treated, and of course the expected mechanical and metallurgical characteristics.
The type of quenching fluid is imposed by the material or heat treatment specifications. Substitutions can be allowed, in this case this possibility is clearly and sometimes explicitly written in the specification ("Another fluid can be proposed..." or "a gas quench can be performed when an air quench is requested"...).
In all cases, the substitution must be approved by the designer of the parts to be treated.
Because of the great difference in characteristics between the various quenching fluids and systems, AMS 2759 requires the heat treatment company to establish and implement a quenching system validation procedure. This applies both to the commissioning of a fluid (e.g. bath replacement) and to the substitution of one fluid for another.
The following paragraphs summarize the main requirements for each type of quenching fluid
Tempering oils
The standards call for oil quenching, but do not specify the type of oil or impose specific characteristics.
Only the bath temperature requirements limit the possibilities to cold quenching oils (UHA normal or UHB fast) and vacuum quenching oils (UHV).
Monitoring the performance of the quenching system is required.
The common requirement of all designers is the periodic measurement of the drasticity. The most common frequency is quarterly.
When the criteria of acceptance, of conformity are not defined. It is the responsibility of the supplier to determine them. The principle is mainly based on the comparison of the measurement with a reference state (results of the same test at the time of the qualification of the process or the commissioning of the quenching bath).
In the case of a treatment required under AMS 2759, if the acceptance limits for drasticity have not been defined by the supplier (and approved by the designer), AMS 2759 defines default limits. These criteria are quite stringent. It is recommended that custom criteria be defined and approved.
When the treatment is to be carried out according to AMS 2759, the frequency of control of the drasticity is semi-annual but with a compulsory quarterly control of the quenching severity (quenching penetration test of the same principle as the corner specimens).
Hardening severity tests and specimens are well defined in AMS2759. Other types of specimens and materials may be used upon approval by the part designer.
The supplier can be exempted from this last test if the heat treatment of the parts is validated by destructive mechanical tests (tensile tests for example).
The supplier must have a procedure defining these tests, and their exploitation (corrective actions in case of drift).
As these methods are comparative, from one measurement to another, once a method is chosen, it must be fixed and used for all subsequent measurements.
Polymer for hardening aluminum alloys
For polymer solution quenching, AMS 2770 requires the use of polymer solutions that comply with AMS 3025 and therefore mandates the use of Poly Alkylene Glycol (PAG) solutions. In the absence of a requirement from the part designer, default concentrations are defined in Table 4 of AMS 2770. The value of these concentrations depends on the type of polymer used (type as defined in AMS 3025), the grade of aluminum alloy treated, its forming method and the cross-section of the part to be treated.
The monitoring of these PAG baths must be done by monitoring the pH of the bath and its concentration. This concentration must be measured weekly by refractometry or viscosity and checked/confirmed quarterly by viscosity.
Polymer for steel hardening
The substitution of oil quenching by quenching in a polymer solution is possible with a validation plan.
Unlike aluminum alloy processing; AMS 2759 does not mandate a particular type of polymer, but the designer can mandate one.
The development of polymer quenching must aim to reproduce the cooling characteristics of the oil pan that we wish to replace, and demonstrate that the mechanical properties obtained are equivalent. To do this, it is necessary to characterize the cooling in an oil pan.
The methods and procedures for monitoring the polymer solution quenching system typically requested by the aerospace industry are:
As for oil, a periodic monitoring of the drasticity of the solution and for AMS 2759, the severity of quenching of the system. A method adapted to the polymer must be chosen (TENSI, DRAYTON,...).
Concentration
The other monitoring recommended by this guide (bacteria, ...) are not required by the AMS standards, may be, for all or part, by the designers, but remains a good practice.
Water quenching
There are no special requirements for water quenching.
Gas quenching
When not imposed by the technical definition of the article, the pressure and the nature of the quenching gas, may require the approval of the designer (according to the standards and procedures of the designing company).
In general, AMS 2769 Index B and the processing standards of all designers require that the gas used should not "deteriorate the material being processed or the furnace". In other words, the quenching gas must be neutral to the articles to be treated, so as to avoid any contamination or degradation of the surfaces of the articles not reworked after treatment.
The use of nitrogen may be subject to certain restrictions depending on the material being processed.
The use of hydrogen, even in a low concentration mixture, must systematically be approved in advance by the designer of the parts to be treated.
The purity of the gases to be used for quenching is usually specified in the applicable heat treatment standard (AMS2769, Pr-0011, PTF-3,...).
Example of a specification for the purity of process gases: Pr-0011 / SAFRAN Standard
This example is about the quality of the gas to be ordered.