TASK 70-41-02-310-003
1 . General.
A.Joints in stainless steel tubing can be torch brazed when repair is authorized by Engine/Shop Manual. Torch brazing of tube joints shall be performed only by personnel qualified by training and experience in the practice of torch brazing.
B.Joints in nickel based alloy tubing (Inconel) cannot be repaired by torch brazing. Repair of this type of tubing must be accomplished by section replacement. The high temperatures required for re-melt and braze filler material flow make torch brazing impractical for nickel-based alloy tubing.
2 . Preparation of Joint.
Subtask 70-41-02-310-031
A.If any part of the joint to be brazed has been used before, all residual filler metal must be removed from it before attempting to braze the joint. The residual metal could prevent proper assembly because of interference. Braze removal can be accomplished by 2 methods: chemical and abrasive.
(1)Chemical Removal.
Braze filler metal can be removed from part small enough to be immersed in an acid bath. This method can also be used by swabbing the acid solution onto large or unwieldy parts that cannot be immersed. Proceed as follows:
(a)Protect workbench, tools, etc. with a plastic dropcloth before mixing or using the acid.
WARNING:
WEAR RUBBER GLOVES AND APRON, AND FACE SHIELD, WHILE HANDLING OR USING THE ACID SOLUTION. IF ACID CONTACTS THE SKIN OR EYES, FLUSH IMMEDIATELY WITH COOL WATER, AND OBTAIN IMMEDIATE MEDICAL AID.
WARNING:
NEVER ADD WATER TO ACID WHEN MIXING SOLUTIONS. HEAT OF REACTION CAN VAPORIZE WATER EXPLOSIVELY, CAUSING BOILING AND SPATTERING.
(b)Prepare a 50 volume percent solution of C04-072   nitric acid by slowly pouring one part of nitric acid (67 to 70 wt %) into one part of water while stirring vigorously.
(c)Immerse the part in solution until residual braze filler metal is dissolved. If immersion is impractical, swab with a plastic cloth or swab affixed to a stainless steel or plastic rod.(Cotton swabs are acceptable for small areas.)
(d)Rinse part thoroughly with water to remove all acid residue.
CAUTION:
SAND CAREFULLY TO AVOID UNDERCUTTING.
(2)Abrasive Removal.
Areas of high residual braze filler metal, such as original braze fillets, can be removed carefully with a silicon carbide sandroll or abrasive. Do not use aluminum oxide. After high metal is removed, the remaining filler metal can be removed manually, using 80 grit abrasive paper or cloth or chemicals, as described in paragraph2.A.(1), above.
B.Gap Clearance.
Trial fit the parts to be joined, and check to be sure that the clearance between faying surfaces does not exceed 0.005 inch(0.13 mm). On round tube joints, the diametral clearance should be 0.002-0.005 inch (0.05-0.13 mm). Too much clearance will prevent the filler metal from bridging the gap, even if both faying surfaces are wetted. If necessary, rework tube ends, couplings or fittings to obtain proper clearance.
C.Surface Cleaning.
It is necessary to clean the surfaces of the joint immediately before brazing to ensure that filler metal will wet the faying surfaces. Flux will not remove oil or grease films, or surface oxides. Further, it is necessary to polish the surface to produce the capillary action required to cause the filler metal to flow into the joint. Clean and polish as follows:
(1)Insert plugs in ends of tubing to prevent entry of abrasive dust.
(2)Polish approximately 1.50 inches (38.1 mm) of each tube end with 180 grit abrasive paper or cloth. Use circumferential polishing strokes, rather than axial or longitudinal, to facilitate the flow of filler metal.
WARNING:
ACETONE AND METHYL-ETHYL-KETONE ARE TOXIC AND EXTREMELY FLAMMABLE. AVOID EXCESSIVE INHALATION OF VAPORS AND CONTACT WITH THE SKIN. TAKE PRECAUTIONS AGAINST FIRE.
(3)Remove abrasive dust completely from part by wiping the polished areas with a clean cloth wetted with acetone or methyl-ethyl-ketone.
(4)Remove plugs, being careful to avoid touching the cleaned areas with bare hands.
3 . Brazing.
Subtask 70-41-02-310-032
WARNING:
DO NOT BRAZE IF YOU ARE NOT APPROVED (TRAINED AND QUALIFIED). USE PERSONAL PROTECTION EQUIPMENT AND APPROVED SAFETY PROCEDURES.
A.Mark the insertion depth of the tube, as specified in the appropriate assembly or repair section, by marking a light scribe mark. Braze as follows:
WARNING:
ACETONE AND METHYL-ETHYL-KETONE ARE TOXIC AND EXTREMELY FLAMMABLE. AVOID EXCESSIVE INHALATION OF VAPORS AND CONTACT WITH THE SKIN. TAKE PRECAUTIONS AGAINST FIRE.
CAUTION:
DO NOT HANDLE CLEANED AREA OF TUBING WITH BARE HANDS. USE CLEAN GLOVES, OR HANDLE ONLY AREAS THAT WILL NOT BE WETTED BY FILLER METAL. USE CLEAN TWEEZERS OR NEEDLE-NOSED PLIERS TO HANDLE RINGS OF FILLER METAL.
(1)Clean rings of filler metal in acetone C04-003   or methyl-ethyl-ketone C04-001  , and install in internal grooves in coupling. Surface oxides on braze filler may be removed mechanically using dry No. 00 grade steel wool. Apply a thin coating of flux to the entire inner surface of the coupling. If the joint is a simple lap joint in which the outer surface of one tube mates with the inner surface of another, lightly coat the entire inner mating surface of the female member with flux.
(2)Coat the cleaned external surface of the male tube member with flux, and insert it in the coupling or mating tube. Make sure that insertion depth, as previously marked on tubes, is correct.
(3)Braze Parameters.
(a)Process - Oxy Acetylene.
(b)Acetylene - 5 psi (34.5 kPa).
NOTE:
Alternative fuel gasses such as MAPP gas or propane are permitted. The operator must be certified with the specific torch and technique and must be able to control the heat input. The operator must demonstrate with coupons that the process will make a good braze joint.
(c)Oxygen - 12 psi (82.7 kPa).
(d)Tip sizes - No. 1 to No. 4 (dependent on stock thickness).
(e)Flame - neutral to slightly reducing.
(f)Filler - AMS 4770 (low temp) or AMS 4772 (high temp).
(g)Flux - AMS 3410 or AMS 3411 for AMS 4770 silver braze alloy.
(h)Flux - AMS 3411 for AMS 4772 silver braze alloy.
CAUTION:
DO NOT USE OXIDIZING FLAME. DO NOT LET THE SMALL LIGHT BLUE CONE OF THE FLAME TOUCH THE METAL. ONLY THE DEEPER BLUE FLAME SHOULD CONTACT THE METAL OR BRAZE ALLOY.
(4)Move the flame around the joint to distribute the heat evenly; stainless steel does not conduct heat readily, and all parts of the joint must be heated directly by the flame. If a coupling with internal rings of filler metal is used, watch for the filler metal to flow to the outer edges of the coupling. In a simple lap joint, force feed the filler metal until flow into the joint is observed and can be seen around the entire periphery of the exposed edge of the joint. Do not pile up excessive filler metal at the exposed edge.
CAUTION:
FLUX RESIDUE IS HIGHLY CORROSIVE AND MUST BE REMOVED COMPLETELY.
(5)Let the joint cool, then remove flux residue with a cloth saturated in warm water and a stainless steel wire brush. Flush the inside with hot water at minimum 150°F (66°C), or steam clean by blowing steam through the tube for 10 minutes minimum. Blow out residual water with filtered shop air. Where feasible, brazed tubes may be placed in boiling water for 10 minutes minimum to dissolve braze flux and blow dry with filtered shop air.
4 . Braze Acceptability Limits for Tubing.
Subtask 70-41-02-310-033
A.Inspect finished joints as follows:
(1)Acceptability Limits for Brazing.
Visual indications are surface discontinuities which can be observed without magnification or aid of a non-destructive test process, that may indicate the presence of surface flaws.
(a)The following acceptability limits apply to the braze filler material and are not applicable to the parent material. If type of imperfection is not assigned specific limits, the imperfection is not permitted.
(b)Exposed Edge Joints.
The following imperfections in exposed edge joints as determined by visual inspection are permitted for all tubing braze joints when silver or gold braze filler material is specified. An exposed edge joint is defined as the external edge of the joint visible from the outside of the tube (See Figure 8).
1Maximum Linear Fillet Length. (See Figure 7).
aFor material thicknesses less than or equal to 0.2 inch (5.08 mm):
DIM A maximum = material thickness.
DIM B maximum = 125 percent of material thickness or 0.10 inch (2.54 mm), whichever is greater.
bFor material thicknesses greater than 0.2 inch (5.08 mm):
DIM A maximum = 0.25 inch (6.35 mm) or material thickness, whichever is less.
DIM B maximum = 125 percent of DIM A.
2Irregular Fillets.
aIrregular fillets (Type A) are defined as non-linear fillet edges where the external braze fillet varies as shown in Figure 6. The fillet length (leg length) may not vary more than fifty percent of the maximum allowed fillet length [Ref para 4.A.(1)(b)1] within 0.25 inch (6.35 mm) of the periphery length for irregular fillet “Type A”.
bIrregular fillets "Type B" are defined as non-linear surfaces of exposed filler material (See Figure 5). A maximum of 0.020 inch (5.08 mm) non-linearity above or below braze material surface is allowed for "Type B" irregular fillets.
3Negative Fillet.
A Negative fillet is defined as a lack of filler material at the edge of the materials being joined by brazing (See Figure 4.) Localized concavity is when negative fillet length is not completely around the braze joint. Negative fillets or localized concavity are not allowed.
4Pin Holes or Surface Porosity in Braze Fillets.
Pin holes or surface porosity is defined as gas pockets or small holes in the surface of the braze material that has a depth which may extend totally through the filler material.
5Acceptability Limits for Pin holes or Porosity.
Pin holes with a maximum diameter of 0.030 inch (0.76 mm) are allowed with a minimum spacing of no greater than 2X the largest adjacent indication with a maximum of six pin holes allowed per linear inch(25.4 mm) of fillet.
(2)Non-Exposed and/or Internal Edge Joints.
Imperfection limits do not apply for internal edge joints. Non-exposed and/or internal edge joints do not require inspection.
(3)Internal Joints Acceptability Limits.
An internal joint is defined as the common mating surfaces of the pieces being joined by braze filler material (See Figure 3).
(a)Aggregate Area.
The total area of voids and unbonded areas compared to the total joint area shall not be greater than 35 percent for stainless steel tubing that utilize silver and gold alloys as braze filler material. The total area of voids and unbonded areas compared to the total joint area shall not be greater than 20 percent for nickel based alloy tubing that utilize nickel alloys as braze filler material.
NOTE:
"Void" is defined as a volume unfilled with braze filler material between the surfaces being joined. "Unbonded area" is defined as an area within a brazed joint in which the braze filler flowed, but did not bond to one or more of the surfaces being joined.
(b)Maximum Length of a Single Imperfection.
The maximum length of a single imperfection compared to the total length (measured in the same direction) of the joint shall not be greater than 35 percent.
(c)Tubing Joints with Pressure Differential.
When the braze joint is exposed to the tube internal pressure and if a pressure check requirement exists, then a continuous sound braze is required for a minimum of 0.060 inch (1.52 mm) in any direction. This requirement takes precedence over the four requirements of paragraph 4.A.(1)(b). The 0.060 inch continuous sound braze may be located any where within the required internal braze area (see Figure 2).
(d)Non-Pressure Differential Joints.
For non-pressure differential joints such as attachments for clamps, wear sleeves and brackets on formed tubing without a pressure differential between the internal and external environment of the joint, requirement for internal joint imperfections need only apply to an area of 0.150 inch (3.81 mm) width around the periphery.
(e)Braze splatter/positive metal flashing and/or droplets of the braze alloy not part of the braze fillet and not exceeding 0.020 inch (0.51 mm) in height above the tube wall and/or fitting surface is acceptable up to 0.50 inch (12.7 mm) away from the braze joint. Braze splatter within 0.50 inch (12.7 mm) of the braze joint that exceeds 0.020 inch (0.51 mm) in height shall be removed to a height of 0.00 - 0.002 inch (0.0 - 0.51 mm) per Subtask 70-41-02-310-031, Preparation of Joint. Remove the splatter to produce a surface finish no greater than 32 microinches (0.85 micrometers) RMS. Visual comparison is an acceptable method for determining height of braze splatter.
Braze splatter not exceeding 0.003 inch (0.8 mm) in height above the wall and/or fitting surface is acceptable if located 0.050 inch (12.7 mm) away from the braze joint. Braze splatter greater than 0.50 inch (12.7 mm) away from the braze joint must be removed to a height of 0.00 - 0.003 inch (0.0 - 0.51 mm) per Subtask 70-41-02-310-031, Preparation of Joint. Remove the splatter to produce a surface finish no greater than 32 microinches (.085 micrometers) RMS.
Braze splatter is not allowed on sealing surfaces or threads of fittings (see Figure 1).
Figure 1   Examples of Mating and Sealing Surfaces
Figure 2   Internal Braze Area
Figure 3   Internal Joint Type
Figure 4   Negative Fillet
Figure 5   Type B Irregular Fillet
Figure 6   Type A Irregular Fillet
Figure 7   Maximum Linear Fillet Length
Figure 8   Examples of Exposed Edge Joints