Different types of Castings defects

  A properly designed casting, a properly prepared mould and correctly malted metal should result in a defect free casting. However, if proper control is not exercised in the foundry-sometimes it is too expensive - a variety of defects may result in a casting.

These defects may be the result of:
(a) improper pattern design,
(b) improper mould and core construction,
(c) improper melting practice,
(d) improper pouring practice and
(e) Because of molding and core making materials.
(f) Improper gating system
(g) Improper metal composition
(h) Inadequate melting temp and rate of pouring
It creates a deficiency or imperfection.

Exceeding quality limits imposed by design and service casting defects are mainly 3 categories. These are:

(1) Major or most severe defects
(2) Intermediate defects
(3) Minor defects

Surface defects:

Due to design and quality of sand molds and general cause is poor ramming.

Blow:

Blow is relatively large cavity produced by gases which displace molten metal form.

 Scar:

Due to improper permeability or venting.A scare is a shallow blow. It generally occurs on flat surf; whereas a blow occurs on a convex casting surface. A blister is a shallow blow like a scar with thin layer of metal covering it,

Scab:

This defect occurs when a portion of the face of a mould lifts or breaks down and the recess thus made is filled by metal. When the metal is poured into the cavity, gas may be disengaged with such violence as to break up the sand which is then washed away and the resulting cavity filled with metal. The reasons can be: - to fine sand, low permeability of sand, high moisture content of sand and uneven moulds ramming.

Drop:

Drop or crush in a mould is an irregularly shaped projection on the cope surface of a casting. This defect is caused by the break-away of a part of mould sand as a result of weak packing of the mould, low strength of the molding sand, malfunctioning of molding equipment, strong jolts and strikes at the flask when assembling the mould.
The loose sand that falls into the cavity will also cause a dirty casting surface, either on the top or bottom surface of the casting, depending upon the relative densities of the sand and the liquid.

Penetration:

It is a strong crust of fused sand on the surface of a casting which results from insufficient refractoriness of molding materials, a large content of impurities, inadequate mould packing and poor quality of mould washes. 

When the molten metal is poured into the mould cavity, at those places when the sand packing is inadequate, some metal will flow between the sand particles for a distance into the mould wall and get solidified. When the casting is removed, this lump of metal remains attached to the casting. Of course, it can be removed afterwards by chipping or grinding.

Buckle:

A buckle is a long, fairly shallow, broad, vee depression that occurs in the surface of flat castings. It extends in a fairly straight line across the entire flat surface.

It results due to the sand expansion caused by the heat of the metal, when the sand has insufficient hot deformation. It also results from poor casting design providing too large a flat surface in the mold cavity.  

Buckling is prevented by mixing cereal or wood flour to sand.

Internal defects:

Blow holes:

Blow holes, gas holes or gas cavities are well rounded cavities having a clean and smooth surface. They appear either on the casting surface or in the body of a casting.

These defects occur when an excessive evolved gas is not able to flow through the mould. So, it collects into a bubble at the high points of a mould cavity ad prevents the liquid metal from filling that space. 

This will result in open blows. Closed, cavities or gas holes are formed when the evolved gases or the dissolved gases in the molten metal are not able to leave the m ass of the molten metal as it solidifies and get trapped within the casting.

These defects are caused by : 

i) excessive moisture content (in the case of green sand moulds) or organic content of the sand, moisture on chills, chaplets or metal inserts, 

ii) inadequate gas permeability of the molding sand (due to fine grain size of sand, high clay content, hard ramming), 

iii) poor venting of mould, insufficient drying of mould and cores, cores not properly vented, high gas content of the molten metal, 

iv) low pouring temperature and incorrect feeding of the casting etc.

Pin holes:

Pin holes are small gas holes either at the surface or just below the surface. When these are present, they occur in large numbers and are fairly uniformly dispersed over the surface. 

This defect occurs due to gas dissolved in the alloy and the alloy not properly degassed.

Visible defects:

Wash:

A cut or wash is a low; projection on the drag face of a casting that extends along the surface, decreasing in height as it extends from one side of the casting to the other end. 

It usually occurs with bottom gating castings in which the molding sand has insufficient hot strength, and when too much metal is made to flow through one gate into the mold cavity,

Rat tail:

A rat tail is a long, shallow, angular depression in the surface of a flat rating and resembles a buckle, except that, it is not shaped like a broad vee. 

The reasons for this defect are the same for buckle.

Hot tear:

Hot tears are hot cracks which appear in the form of irregular crevices with a dark oxidized fracture surface. They arise when the solidifying met does not have sufficient strength to resist tensile forces produced during solidification. 

They are chiefly from an excessively high temperature of casting metal, increased metal contraction incorrect design of the gating system and casting on the whole (causing portions of the casting to be restrained from shrinking freely during cooling which in turn causes excessive high intern resistance stresses), poor deformability of the cores, and non-uniform cooling which gives rise t internal stresses. This defect can be avoided by improving the design of the casting and by having a mould of low hot strength and large hot deformation.

Shrinkage:

A shrinkage cavity is a depression or an internal void in a casting that results from the volume contraction that occurs during solidification.

Swell:

A swell is a slight, smooth bulge usually found on vertical faces of castings, resulting from liquid metal pressure. It may be due to low strength of mould because of too high a water content or when the mould is not rammed sufficiently.

Shift:

Mold shift refers to a defect caused by a sidewise displacement of the mold cope relative to the drag, the result of which is a step in the cast product at the parting line.

Core shift is similar to mold shift, but it is the core that is displaced, and (he dis-placement is usually vertical. Core shift and mold shift are caused by buoyancy of the molten metal 

Misrun or cold sheet or short run:

This defect is incomplete cavity filling. The reasons can be: - inadequate metal supply, too- low mould or melt temperature, improperly designed gates, .or length to thickness ratio of the casting is too large. When molten metal is flowing from one side in a thin section, it may loose sufficient heat resulting in loss of its fluidity, such that the leading edge of the stream may freeze before it reaches the end of the cavity.

Different Types of Welding Defects

 Identifying common welding defects

Preventing and fixing problems with your welds

If I could wave my magic filler rod and make it so that all your welds would be strong, clean, and sharp-looking, I’d certainly do it. But the last time I went to the welding supply shop and asked to buy a magic filler rod, they looked at me like I was nuts, so for now you just have to live with the fact that some of your welds will be imperfect. Don’t worry about it too much; after all, no welder is perfect, and welding can be a tricky endeavor. What I can do is fill you in on some of the most common weld flaws so that when they show up, you realize that you’re dealing with the same kinds of challenges that hundreds of thousands of welders have cursed and spat about since the first guy figured out how to strike an arc. These are the kinds of defects that you’re likely to notice only after you’ve finished a weld (either a single pass or a complete weld, depending on the defect). Most are pretty easy to detect, and — thankfully — relatively easy to adjust for and prevent. To help prevent welding defects before they happen, be sure your welding materials are clean and in good shape before you start a project. The metal you’re planning to weld should be free of any material that may contaminate the weld. Remove any grease, paint, or oil from the metal. You should also put the pieces to be welded in place to make sure they fit together and line up properly.

 Most Common Types of Welding Defects

 Incomplete Penetration

Incomplete penetration happens when your filler metal and base metal aren’t joined properly, and the result is a gap or a crack of some sort. Check out the Figure below for an example of incomplete penetration.

a common case of incomplete penetration

Welds that suffer from incomplete penetration are weak at best, and they’ll  likely fail if you apply much force to them. (Put simply, welds with incomplete  penetration are basically useless.)
Here’s a list of the most common causes of incomplete penetration welding defect.

The groove you’re welding is too narrow, and the filler metal doesn’t
reach the bottom of the joint.

✓ You’ve left too much space between the pieces you’re welding, so they
don’t melt together on the first pass.
✓ You’re welding a joint with a V-shaped groove and the angle of the
groove is too small (less than 60 to 70 degrees), such that you can’t
manipulate your electrode at the bottom of the joint to complete
the weld.
✓ Your electrode is too large for the metals you’re welding.
✓ Your speed of travel(how quickly you move the bead) is too fast, so
not enough metal is deposited in the joint.
✓ Your welding amperage is too low.If you don’t have enough electricity
going to the electrode, the current won’t be strong enough to melt the
metal properly

 Incomplete Fusion

Incomplete fusion occurs when individual weld beads don’t fuse together, or  when the weld beads don’t fuse properly to the base metal you’re welding,  such as in below.

a textbook example of incomplete fusion

The most common type of incomplete fusion is called overlap and usually  occurs at the toe(on the very top or very bottom of the side) of a weld. One of the top causes is an incorrect weld angle, which means you’re probably holding the electrode and/or your filler rods at the wrong angle while you’re making a weld; if you think that’s the case, tweak the angle a little at a time until your overlap problem disappears.

Here are a few more usual suspects when it comes to incomplete fusion
causes.

✓ Your electrode is too small for the thickness of the metal you’re welding.
✓ You’re using the wrong electrode for the material that you’re welding.
✓ Your speed of travel is too fast.
✓ Your arc length is too short.
✓ Your welding amperage is set too low.
If you think your incomplete fusion may be because of a low welding amperage, crank up the machine! But be careful: You really need only
enough amperage to melt the base metal and ensure a good weld.
Anything more is unnecessary and can be dangerous.
✓ Contaminants or impurities on the surface of the parent metal(the metal
you’re welding) prevent the molten metal (from the filler rod or elsewhere
on the parent metal) from fusing.

Undercutting

Undercutting is an extremely common welding defect. It happens when your  base metal is burned away at one of the toes of a weld. To see what I mean, look at Figure.

When you weld more than one pass on a joint, undercutting can occur between the passes because the molten weld is already hot and takes less heat to fill, yet you’re using the same heat as if it were cold. It’s actually a very serious defect that can ruin the quality of a weld, especially when more than 1⁄32 inch is burned away. If you do a pass and notice some undercutting, you must remove it before you make your next pass or you risk trapping slag (waste material — see the following section) into the welded joint (which is bad news). The only good thing about undercutting is that it’s extremely easy to spot after you know what you’re looking for.

Here are a few common causes of undercutting:
✓ Your electrode is too large for the base metal you’re welding.
✓ Your arc is too long.
✓ You have your amperage set too high.
✓ You’re moving your electrode around too much while you’re welding.
Weaving your electrode back and forth is okay and even beneficial, but if
you do it too much, you’re buying a one-way ticket to Undercutting City
(which is of course the county seat for Lousy Weld County).

Slag Inclusions

A little bit of slag goes a long way . . . toward ruining an otherwise quality weld. Slagis the waste material created when you’re welding, and bits of this solid material can become incorporated (accidentally) into your weld, as in Figure . Bits of flux, rust, and even tungsten can be counted as slag and can cause contamination in your welds.

Common causes of slag inclusions include

✓ Flux from the stick welding electrode that comes off and ends up in the
weld
✓ Failure to clean a welding pass before applying the next pass
Be sure to clean your welds before you go back in and apply a second weld bead.
✓ Slag running ahead of your weld puddle when you’re welding a V-shaped
groove that’s too tight
✓ Incorrect welding angle
✓ Welding amperage that’s too low

Flux Inclusions

If you’re soldering or brazing (also called braze welding), flux inclusions can be a real problem. If you use too much flux in an effort to “float out” impurities from your weld, you may very well end up with flux inclusions like those in Figure . (Head to Chapter 13 for more on brazing and soldering.)

If you’re working on a multilayer braze weld, flux inclusion can occur when you fail to remove the slag or glass on the surface of the braze before you apply the next layer. When you’re soldering, flux inclusion can be a problem if you’re not using enough heat. These inclusions are usually closely spaced, and they can cause a soldered joint to leak. If you want to avoid flux inclusions (and believe me, you do), make sure you do the following:
✓ Clean your weld joints properly after each pass.This task is especially
important when you’re brazing.
✓ Don’t go overboard with your use of flux.
✓ Make sure you’re using enough heat to melt the filler or flux material.

Porosity

If you read very much of this book, you quickly figure out that porosity(tiny holes in the weld) can be a serious problem in your welds (especially stick or mig welds). Your molten puddle releases gases like hydrogen and carbon dioxide as the puddle cools; if the little pockets of gas don’t reach the surface before the metal solidifies, they become incorporated in the weld, and nothing can weaken a weld joint quite like gas pockets. Take a gander at Figure  for an example of porosity.

Following are a few simple steps you can take to reduce porosity in your
welds:
✓ Make sure all your materials are clean before you begin welding.
✓ Work on proper manipulation of your electrode.
✓ Try using low-hydrogen electrodes.

Cracks

Cracks can occur just about everywhere in a weld: in the weld metal, the plate next to the weld metal, or in any other piece affected by the intense heat of welding. Check out the example of cracking in Figure.

Here are the three major types of cracks, what causes them, and how you can prevent them.

✓ Hot cracks:

This type of crack occurs during welding or shortly after you’ve deposited a weld, and its cause is simple: The metal gets hot too
quickly or cools down too quickly. If you’re having problems with hot cracking, try preheating your material. You can also postheat your material, which means that you apply a little heat here and there after you’ve finished welding in an effort to let the metal cool down more
gradually.

✓ Cold cracks:

This type of crack happens well after a weld is completed and the metal has cooled off. (It can even happen days or weeks after a
weld.) It generally happens only in steel, and it’s caused by deformities in the structure of the steel. You can guard against cold cracking by
increasing the thickness of your first welding pass when starting a new weld. Making sure you’re manipulating your electrode properly, as well as pre- and postheating your metal, can also help thwart cold cracking.

✓ Crater cracks:

These little devils usually occur at the ending point of a weld, when you’ve stopped welding before using up the rest of an
electrode. The really annoying part about crater cracks is that they can cause other cracks, and the cracking can just kind of snowball from
there. You can control the problem by making sure you’re using the appropriate amount of amperage and heat for each project, slowing your
speed of travel, and pre- and postheating.

Warpage

If you don’t properly control the expansion and contraction of the metals you work with, warpage(an unwanted distortion in a piece of metal’s shape) can be the ugly result. Check out an example in Figure.

If you weld a piece of metal over and over, the chances of it warping are much higher. You can also cause a piece of metal to warp if you clamp the joints too tightly. (If you allow the pieces of metal that make the joint to move a little, there’s less stress on them.)
Say you’re welding a Tjoint. The vertical part of the Tsometimes pulls itself toward the weld joint. To account for that movement, simply tilt the vertical part out a little before you weld, so that when it tries to pull toward the weld joint, it pulls itself into a nice 90-degree angle!
The more heat you use, the more likely you are to end up with warpage, so be sure to use only the amount of heat you need. Don’t overdo it. Opting for a slower speed of travel while welding can also help to cut down on warpage.

Spatter

Spatter(small particles of metal that attach themselves to the surface of the material you’re working on.) is a fact of life with most kinds of welding; no matter how hard you try, you’ll never be able to cut it out completely. You can see it in all its glory in Figure 11-5 in Chapter 11.
You can keep spatter to a minimum by spraying with an anti-spatter compound (available at your welding supply store) or by scraping the spatter off the parent metal surface.

These are the 10 most commonly found welding defects. Some new and useful articles are coming soon. So stay tuned.