1、_ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising there
2、from, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions. Copyright 2012 SAE International All rights reserved. No part of this publication m
3、ay be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-4970 (outside U
4、SA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/AMS2678A AEROSPACE MATERIAL SPECIFICATION AMS2678 REV. A Issued 2006-08 Revised 2013-02 Super
5、seding AMS2678 Furnace Brazing of Parts in a Vacuum Atmosphere RATIONALE AMS2678A includes changes to 3.2 to list acceptable gases for partial pressure atmosphere, and changes to 3.5.2 and 3.5.2.2 detailing requirements for load thermocouples. 1. SCOPE 1.1 Purpose This specification covers the requi
6、rements and procedures for the brazing of parts in a very low pressure gas which referred to as “vacuum” herein. 1.2 Application This process provides general requirements and practices for vacuum brazing carbon and alloy steels, corrosion resistant steels, precipitation hardening alloys, nickel and
7、 cobalt-based alloys, aluminum alloys, and titanium alloys, but usage is not limited to such applications. For specific brazing requirements for the materials to be brazed, the applicable AMS, AWS, or other engineering specification should be referenced. 1.2.1 When there is a conflict between the re
8、quirements of this braze specification and the applicable specific brazing specification, requirements of the specific brazing specification shall take precedence. 1.3 Safety - Hazardous Materials While the materials, methods, applications, and processes described or referenced in this specification
9、 may involve the use of hazardous materials, this specification does not address the hazards which may be involved in such use. It is the responsibility of the user to ensure familiarity with the safe and proper use of any hazardous materials and to take necessary precautionary measures to ensure th
10、e health and safety of all personnel involved. 2. APPLICABLE DOCUMENTS The issue of the following documents in effect on the date of the purchase order forms a part of this specification to the extent specified herein. The supplier may work to a subsequent revision of a document unless a specific do
11、cument issue is specified. When the referenced document has been cancelled and no superseding document has been specified, the last published issue of that document shall apply. SAE AMS2678A Page 2 of 8 2.1 SAE Publications Available from SAE International, 400 Commonwealth Drive, Warrendale, PA 150
12、96-0001, Tel: 877-606-7323 (inside USA and Canada) or 724-776-4970 (outside USA), www.sae.org. AMS2750 Pyrometry AMS2769 Heat Treatment of Parts in a Vacuum ARP1962 Training and Approval of Heat-Treating Personnel 2.2 NFPA Publications Available from National Fire Protection Association, 1 Batteryma
13、rch Park, Quincy, MA 02169-7471 or www.nfpa.org. NFPA 86 Standard for Ovens and Furnaces 3. TECHNICAL REQUIREMENTS 3.1 Furnace Equipment The furnace components shall not deleteriously affect material processed during a normal production run. Material may be affected, for example, by physical or chem
14、ical reaction between the parts and the furnace components or gases used to maintain partial pressure in the furnace. See also 3.5.3.1. 3.1.1 Vacuum System The vacuum pumping system shall have sufficient pumping capacity to evacuate the furnace to a pressure within the recommended range for the mate
15、rials being processed, and to maintain the required pressure during the entire brazing process, including any periods when a partial pressure is maintained using a controlled flow of gas. See also 3.2. 3.1.1.1 Vacuum Sensing Equipment The vacuum furnace shall be equipped with at least one gauge capa
16、ble of sensing and recording the pressure in the vacuum heating chamber of the furnace. Vacuum sensing equipment shall be calibrated in accordance with the manufacturers specification. Gauge types in Table 1 are recommended for the pressure ranges shown. TABLE 1 - PRESSURE VERSUS RECOMMENDED GAUGE T
17、YPE Pressure (Microns) Gauge 1 to 103Thermocouple or Pirani 10-6to 1 Hot filament ionization 10-4to 10 Cold cathode ionization 3.1.1.2 Vacuum Gauge Calibration The method of vacuum gauge calibration shall be in accordance with the manufacturers specification and shall be acceptable to the purchaser.
18、 For furnaces used for processing materials at pressures of 1 micron or lower, vacuum gauges shall be calibrated every 3 months; otherwise, vacuum gauges shall be calibrated at least annually, unless conditions necessitate more frequent calibrations. Alternatively, a frequency may be established by
19、periodic testing and agreed to by the purchaser. SAE AMS2678A Page 3 of 8 3.1.1.3 Leak Rate Testing for gas leakage shall be performed at ambient or elevated temperature. Initial leak rate shall be determined after closing the vessel and evacuating to at least 50 microns. After reaching the initial
20、evacuation level set point for the process being performed, all valves to the vessel chamber shall be closed, the initial pressure recorded, and a second reading of pressure made a minimum of 15 minutes after the first reading. Leak rate shall then be determined by dividing the rise in pressure (dif
21、ference between final and initial readings) by the time in hours. Leak rate shall be expressed as microns per hour. Permissible leak rate shall be in accordance with AMS2769 or as specified by the purchaser. 3.1.1.4 If the leak rate test fails, the furnace may be baked out as specified in 3.1.1.5 an
22、d retested. 3.1.1.5 Bake-Out Cycle The furnace shall be subjected to a bake-out (clean up) cycle monthly or when one or more of the following conditions occurs: The furnace fails a periodic leak rate test as in 3.1.1.3. After cycles where aluminum brazing filler metals are used and other filler meta
23、l brazing cycles are to follow. Before running titanium base metals or filler metals containing titanium. 3.1.1.5.1 The bake-out cycle shall be performed at a minimum of 50 F (28 C) above the brazing temperature to be used on the next series of brazing cycles. 3.1.1.5.2 Bake-out shall be for a perio
24、d of 1 hour minimum at the lowest achievable furnace pressure, and until the pressure is within the brazing range specified by the cognizant engineering organization or no change in pressure is observed over a period of 30 minutes. 3.1.1.5.3 Following the bake-out period, the furnace shall be cooled
25、 to a temperature lower than the brazing temperature, and the leak rate measured using the method in 3.1.1.3. 3.1.2 Pyrometry shall conform to AMS2750. 3.1.2.1 Thermocouples Thermocouple types K, N, and Nickel-Nickel Molybdenum shall be suitably protected by enclosing them in metal or ceramic sheath
26、s, and shall be of a type compatible with the range of temperatures and vacuum conditions used. They shall be of a suitable size and located in such a way that the sheaths receive direct radiation from the heating elements and furnace walls. 3.1.2.1.1 The thermocouple types shown in Table 2 shall be
27、 used in vacuum for furnace control or temperature uniformity test measurements unless otherwise approved by the cognizant engineering organization. SAE AMS2678A Page 4 of 8 TABLE 2 - THERMOCOUPLE TYPES VERSUS MAXIMUM OPERATING TEMPERATURE Type Maximum Temperature F Maximum Temperature C K (Chromel-
28、Alumel) 2200(1)1204 N (Nicrosil-Nisil) 2200(1)1204 Nickel - Nickel/Molybdenum 2300 1260 S, R (Platinum/Rhodium - Platinum/Rhodium) 2600 1427 B (Platinum/Rhodium - Platinum/Rhodium) 3100 1704 Tungsten -Tungsten/Rhenium 4000 2204 Note: (1) These thermocouples shall be discarded or recalibrated after b
29、eing exposed to any of the following service conditions: five (5) brazing cycles or accumulation of 5 hours service at or above 2100 F (1149 C) or ten (10) brazing cycles or accumulation of 10 hours service between 2000 F (1093 C) and 2100 F (1149 C) or, for cycles below 2000 F (1093 C), the applica
30、ble requirement of AMS2750 shall apply. 3.1.2.1.2 When metal or ceramic sheaths are used, the processor shall ensure that no chemical reaction between the sheath or insulation in contact with the thermocouple and the thermocouple wire can occur. 3.1.3 Quenching Furnaces having quenching capability s
31、hall be equipped with cooling means sufficient for the material, components, load and process being performed. 3.1.3.1 Gas Quenching When gas quenching is specified, it shall be accomplished by backfilling the furnace with a gas that has no detrimental metallurgical effects on the material being pro
32、cessed or on the furnace equipment. The system and the pressure of the backfill gas selected shall be capable of cooling the parts at a rate sufficient to meet the material property requirements specified by the cognizant engineering organization. 3.1.3.1.1 When gas quenching is specified, the load
33、shall be vacuum-cooled to below the solidus of the braze alloy before the furnace is backfilled for quenching. 3.1.3.2 Cooling Where slow cooling is required following the brazing step, vacuum cooling or cooling with a partial pressure of inert gas is permitted. Controlled cooling using a programmed
34、 heat input is permitted provided load thermocouples are used to measure the actual load temperature during the cooling period. Load thermocouples shall be shielded by the load. Alternatively, working thermocouples may be used to control the cooling by establishing the load cooling time in preproduc
35、tion testing (3.5.2). 3.1.3.3 Use for Heat Treatment When the braze heating/cooling cycle is to be used to accomplish part of a heat treatment process, the furnace must also meet the requirements of the applicable heat treat specification. 3.2 Partial Pressure Atmospheres To minimize metal evaporati
36、on, partial pressures may be maintained during the braze heating cycle, using a flow of inert gas, hydrogen, or mixtures of inert gas and hydrogen. 3.2.1 Hydrogen shall only be used as a partial pressure gas with the approval of the cognizant engineering organization. 3.2.1.1 The use of hydrogen in
37、a vacuum furnace shall be in accordance with NFPA 86. SAE AMS2678A Page 5 of 8 3.2.2 When partial pressures are used to maintain chamber pressures above 100 microns, diffusion pumps shall be isolated from the main vacuum chamber to prevent back streaming. 3.3 Outgassing If the pressure rises during
38、the heat-up cycle to a level such that either the partial pressure control set point is exceeded, causing the gas to stop flowing, or the vacuum level needed to maintain diffusion pump operation is exceeded, then excessive outgassing is occurring. In this instance, the furnace shall be held at a con
39、stant temperature, or the heating rate reduced until the pressure is reduced to within the required range. 3.4 Cleaning Parts, fixtures and materials other than flux or paste, charged into the heating chamber shall be free of ingredients or contaminants which might evaporate and react with the mater
40、ial being brazed or the furnace components. Handling of cleaned parts and fixtures shall be such as to prevent contamination prior to charging into the furnace. 3.4.1 Drying of Paste Parts assembled with braze paste shall have the paste dried prior to being placed in the vacuum furnace. 3.5 Brazing
41、shall be performed in accordance with the requirements of the applicable brazing specification and as follows: 3.5.1 Part Loading Parts and fixtures shall be arranged in the furnace to promote uniform heating and cooling, and when quenching is required, to permit uniform circulation of the quench me
42、dium. 3.5.1.1 Parts shall be arranged so that braze filler metal cannot contact components of the furnace, fixtures, or thermocouples. When ceramic or other materials are used to separate parts, or fixtures from parts, they shall be of a type compatible with the brazing conditions and shall not inte
43、rfere with uniform heating or cooling of the parts. 3.5.2 Load Thermocouples A minimum of two load thermocouples shall be used for procedure qualification. The thermocouples shall be located in the portions of the load that are predicted to be the last to attain the desired temperature. The thermoco
44、uples may be attached to the outer surface of the parts. If a thermocouple is placed in a hole to measure the core or interior of the part being processed, the thermocouple or end of the sheath or well shall make intimate contact with the surface at the bottom of the hole. To avoid errors due to con
45、duction along the length of the thermocouple, the minimum depth of insertion of the thermocouple shall be at least ten times the diameter of the thermocouple. 3.5.2.1 Bare load thermocouples (unsheathed) may be used, provided they are used for only one brazing cycle and then discarded. Thermocouple
46、types shall be selected to ensure that no chemical reaction with the material being brazed or the braze filler metal can occur. 3.5.2.2 In the event a load thermocouple fails, the qualification run need not be aborted as long as another load thermocouple continues to record the correct temperature.
47、3.5.2.3 Once a load has been qualified with load thermocouples, subsequent loads can be run without load thermocouples provided adequate records detailing the number of parts and distribution in the first qualified load are kept on file, and provided that subsequent loads have an equal or fewer numb
48、er of parts in the load, and the distribution of the parts is the same as the distribution in the first load. 3.5.2.4 When use of load thermocouples is impractical, such as with two or three chamber oil- or gas-quench furnaces, tests shall be conducted to establish the correct heat-up time for the l
49、oad. SAE AMS2678A Page 6 of 8 3.5.3 Fixturing Part fixtures shall provide adequate support and positioning of parts to promote uniform heating and cooling. 3.5.3.1 Fixture materials shall be of adequate strength to support the parts being brazed, and shall not react with the material being brazed or the braze filler metals. In particular, fixture/material combinations that readily form eutectics are not permitted. 3.5.4 Braze fil
