Prepared by:

A. SchaffeldHi TecMetal Group, Incorporated

Contents

Introduction 450

Applicable BrazingProcesses 450

Preparation of CastIron for Brazing 450

MetallurgicalConsiderations 451

Brazing Filler Metals 451

Preheating 452

Joint Clearance 452

Surface Plating 453

Heating and Cooling 453

Brazing ofDissimilar Metals 453

Other Applications 454

CHAPTER 23

Photograph courtesy of Modile Manufacturing Company

CAST IRON

AWS BRAZING HANDBOOK 449

INTRODUCTION

450 CHAPTER 23—CAST IRON AWS BRAZING HANDBOOK

The processes used for the brazing of cast ironsare the same as those used for the brazing of steel—furnace, torch, induction, and dip brazing. Like forother metals, the selection of a brazing process forcast iron largely depends on the size and shape of theassembly, the quantity of assemblies to be brazed,and the equipment available. Many applicationsrequire that gray, malleable, and ductile cast irons bebrazed either to themselves or to other metals. Whitecast irons are seldom brazed.

The brazing of gray, ductile, and malleable castirons differs from the brazing of steel in two princi-pal respects. First, a special precleaning method isnecessary to remove graphite from the surface of theiron. Second, the brazing temperature should be keptas low as feasible to avoid a reduction in the hard-ness and strength of the iron.

APPLICABLE BRAZING PROCESSES

All of the processes described in Part 2 of this vol-ume are applicable to the brazing of cast irons. Thechoice of brazing process depends upon the (1) met-als being joined, (2) the brazing filler metals used, (3)the design of the joint, and (4) the relative masses ofthe assemblies to be jointed. Processes that can beused with automatic temperature control are desired,and overheating should be avoided.

Furnace brazing is particularly suited for the pro-duction of high-volume assemblies and the brazingof dissimilar metals. Copper and nickel brazing fillermetals are generally used for these applications. Nor-malizing following brazing is sometimes performedto restore the original properties of the cast iron.

For applications in which little or no decrease instrength can be tolerated, it is mandatory to use abrazing filler metal with the lowest possible flowtemperature. The brazing temperature and cycle timeneed to be minimized to limit the impact on thestructure and properties of the cast iron.

PREPARATION OF CAST IRON FOR BRAZING

The machining of cast iron smears the cut surfaceswith graphite, making it difficult to wet the surfaceswith the brazing filler metal. During brazing, silverand copper brazing filler metals fail to wet the graph-ite flakes or nodules. Thus, the graphite must beremoved from the surfaces prior to brazing. Brazingfiller metals that contain chromium, titanium, orother carbide formers will wet and bond to thegraphite. Brazing filler metals such as AWS BNi-2can be used without the normal cleaning process toremove the graphite.

Some high-silicon cast irons may have siliconoxide on the surface of the casting, which also pre-vents wetting by the brazing filler metal. These sur-faces must be cleaned using one of the methodsdescribed below. When using metals that do not con-tain carbide formers, smeared graphite and graphiteflakes must be removed from the surfaces to bebrazed. Several processes may be used, depending onthe brazement’s application and the cost.

TREATMENT BY FLAMEAn oxidizing oxyacetylene flame is normally

applied to heat the surface and oxidize the surface

CAST IRONCHAPTER 23