Secrets of professional cast iron welding and other repair technologies

Why is cast iron so difficult to weld? It requires pre-heat above 900 deg. F. in an oven environment to properly stress relieve the HAZ. It cracks to relieve weld stress if welded to cold. It hardens if it cools to fast during cool down from 1800 down to 1200 deg. F. Exposure to high temperatures over time graphitizes the iron to the point it no longer melts easily during a welding process.

Properties of Cast Iron(gray iron)High carbon content (over- saturated)

Yield and tensile point the same Easily heat treated to 65 Rc.

Cant stretch or bend to relieve stress below 1200 deg. F.

Carbon ContentApproximately 3%Over-saturated, free carbon, flake formFree carbon absorbs vibration and contributes to other beneficial characteristicsFree carbon prevents it from stretching and bendingAttached carbon is responsible for hardening

Yield Point DifferentialCast iron: Yield = Tensile Cant bend or stretch without crackingThe yield point (YP) of steel (and many other metals) is lower than the tensile point (differential) which allows the metal to bend and stretch without crackingYP differential

Heat TreatmentHeat treating or hardening requires carbon.The metal is heated above its critical temperature and then cooled rapidly to cause hardening.Tempering follows to toughen and set the exact hardness.

CrackingCast iron cracks when strain reaches its tensile point.

Strain can increase until it reaches the tensile point in cast iron with no warning. In steel, weld strain can be relieved when it reaches its yield point and stretching can occur without cracking.

Cast iron requires high temperature preheat to create an artificial yield point and allow stretching.

How cast iron responds to localized heatCrackingHardeningLocalized distortion

Heat related cracksStress is caused by the castings response to restricted expansion and contraction.

Cracks are created when the stress is relieved.

Understanding Expansion and ContractionFree expansion with free contraction

Restricted expansion with free contraction

Restricted expansion with restricted contraction

Cracks are Caused by Expansion/Contraction Stress Its the heat, not the welding method or filler material.Without preheating above 900 deg F. expansion/contraction stress will occur if welding is done in the middle of the casting.

Electric Welding Causes the Most DamageIt simply heats and cools too fast.

Stress Is Avoided by Preheating and Expanding the CastingFusion welding requires preheating to 1500 deg. F.

Furnace brazing requires preheating to 900 deg. F.

Powder fusion welding requires preheating to 1200 deg. F.

If Cracking Occurs in or near a weld, the Casting Was Not Hot Enough!

An Example of Unsuccessful PreheatingLarge pump casing

Insulated and electrically pre-heated to 600 deg. F.

Electric welding with nickel welding rodfailed and caused even more cracks

Localized low temp pre-heat usually makes things worse

This 48 flange had to be replaced with a new steel flange

A portable mill was attached to the discharge nozzle to machine the end flat

A key mill was used to cut slots in the end of the nozzle to accept locking lugs to strengthen the repair

The new flange was attached and sealed with stitching pins only

Stress Cannot Be Caused by Welding or Heating a Corner or End Section of the Casting

HardeningCast iron has a high carbon content and is easily heat treated.

Rapid cooling causes hardening.

Hardening reduces strength and makes cast iron brittle.

Critical TemperatureThe temperature required to harden cast iron is 1200 deg. F. followed by rapid cooling.

The temperature required to anneal or soften cast iron is 1500 degrees for two hours and slooowww cooling for 24 hours.

Electric WeldingElectric welding just happens too fast.

Hardening almost always occurs at the HAZ and not in the weld.

Electric welding in the middle of the casting usually results in cracking

Electric welding on an end or corner never causes cracks but hardening occurs if the preheat temperature is not high enough. (1200 F.)

Its the Heat!

The Bottom LineIf it cracked, it was too cold.

If it got hard, it cooled too fast.

Avoid electric welding on cast iron if at all possible.

The following is an example of the correct way to braze a cast iron part to prevent cracking and hardening.

It works every time!

15,000 pound hydrogen gas compressor cylinder

Damage caused by air-arc to remove a press-fit liner

The gouge occurred during liner removal measured wide X deep X 36 long

A heavy steel stand was fabricated to hold the cylinder in an inclined position

Installing insulating blankets

Oven construction completed

Initial warming with gas torches

The cylinder was preheated to 1000 deg F. over a 24 hour periodto prepare it for brazing

Brazing process using an acetylene torch

Brazing process required about 48 hours including preheat and post heat.

Rough grinding the bronze

High temperature brazing anneals the cast iron and is easily machined

After final machining

Metal Stitching is a cold mechanical process for repairing cast iron without all of the issues associated with welding

When to choose Metal Stitching instead of a welding or brazing process.Preheating the complete part is not practicalDistortion is not acceptableRe-machining is not possible or practicalDowntime is criticalCost When metal stitching will produce the best repair

How Metal Stitching Works

Stitching pins are installed in an overlapping pattern along the crack and Locks go across the crack

The three parts of Metal Stitching Stitching pins Locks Cracked bolt hole repair inserts

Stitching PinsPreferred Pins have a thread design that pulls the sides of the crack together. This enables you to create strength and seal over the entire length of the crack.

Locks Locks are installed across the crack, into a precision drilled hole pattern. They are stacked on top of each other to accommodate different thickness of castings.

Threaded hole repair inserts This bolt hole repair insert has the ability to repair stripped threads and cracks at the same time.

Example of Metal Stitching on a compressor that operates at 500 psi

Gas compressor with a large section blown out by trapped liquid.

The blown out piece is fitted back in place

The piece is clamped firmly in place

The joint is treated like a crack

Stitching pins are installed into the joint to secure the piece

A series of overlapping pins fill the joint creating strength and high pressure seal

After the stitching pins are installed, Lock patterns are drilled across the joint at intervals to accept Locks for strength

The repair is completed and ground flush to the original surface of the casting

All gasket surfaces are re-machined

The repaired hole is machined back to original dimensions

Special thread inserts are installed at the cracked bolt holes to add strength and to seal the holes

Discovering, understanding and solving the cause of the damage before doing the repair is the only way to assure success!

Process for designing a successful repair.Determine if the damage occurred during normal operating conditions or was it caused by an accident or incident.If the damage occurred during normal operating conditions, determine how to prevent the same problem from occurring again or how to reinforce the part to make it stronger than new.

Successful repairs to cast iron that work every time

Fusion welding at 1500 deg F. preheat

Turbo housing brazed at 1000 deg F. preheat

Metal spraying at 200 deg F.

Gary Metal Stitching on the US Capitol Dome made of Cast IronNo heat!

Questions?

Presented by Gary J. Reed CEOLOCK-N-STITCH Inc.

800-736-8261

For more information:www.locknstitch.comwww.fulltorque.com

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