Excerpt from the January 2017 article in International Journal of Engineering Research and Technology by M.R. Latte and P.D. Chougule.
In this research paper, the blow-hole defect analysis of MH1-VST 4-cylinder block fitted on tractors is presented as a case study. An MHI-VST 4-cylinder block is the central component of a tractor or any vehicle. It plays an important role in lubrication, temperature control and stability of the engine. It must be of the highest quality so there is no room for shortcuts. In this case study, parts pre- and post-analysis is done by using quality control (QC) tools such as Pareto charts and cause/effect diagrams. A DMAIC approach is followed and proper actions are taken to reduce the defect and rejection rate at the factory end. The work is done under two phases:
Phase 1: identifying drastic effects of blow holes in production;
Phase 2: sorting out the issues by using various QC tools and expert opinion.
Castings are used to manufacture complex shapes. The castings are bound to have one or more defect. The presence of defects may subject a casting to rejection. The defect causes stress concentration. More time and money is saved if hundred percent perfect castings are made. Defects are minimized by taking precautionary measures in the casting processes.
The same is the case in MHI-VST 4-Cylinder block. There are many possible defects in this casting. Ther are four potential defects: blow hole, core crack, sand drop and shrinkage which has the maximum effect on the rejection percentage. The defects were identified by past production lots and reports provided by the quality department.
The MHI-VST 4-Cylinder block is the central component. It has to be highest possible quality so that it can perform its vital role in the operation of cylinder heads, timing case, sump and flywheel. MHI-VST 4-Cylinder blocks are specially designed to withstand a variety of temperatures and loads to maintain the stability and lubrication of each individual engine. Each block has a number of oil galleries to transfer oil throughout the engine, which maintains the lubrication of all critical components. The block also contains the water galleries needed to provide engine cooling to maintain its optimum operating temperature. The MHI-VST 4-cylinder block unit includes cylinder walls, coolant passages & cylinder sleeves.
1.1 Blowholes in Sand Casting
There are different types of defects produced in sand casting. A high proportion of casting defects are caused due to evolution of gases. One of the major casting defects caused due to gases is holes (gas holes). Gas holes are pinholes and blowholes. This designation belongs to size of the hole and not its origin. Blowhole is very prevalent because of casting scrap. Figure 1 shows a schematic of blowholes, showing blowholes near the core, surface blowholes and casting with blowholes.
The blowholes are smooth walled cavities, essentially spherical, often not contacting the external casting surface. The largest cavities are often isolated. In specific cases, the casting surface can be strewn with blowholes. The interior walls of blowholes can be shiny, more or less oxidized or in case of cast iron can be covered with a thin layer of graphite. Figure 2 shows some slag blowholes having smooth surface and slag accumulated on smooth surface. 
The blowholes are usually revealed by machining or by heavy shot blasting. The defect may take the form of well-defined, bubble-shaped cavities beneath the surface of the casting. These forms of holes may arise from entrapment of more than one sort of gas during the course of mold filling and solidification. It is important to know the origin of and reactions producing these gases so the correct diagnosis and cure can be implemented.
1.2 Types of Blow-holes
When the hot metal is poured inside the sand mold, sand and sand contents gets heated and large amounts of gases are produced inside the casting. The main gas producing processes in the mold are:
a) Rejection of dissolved gases from the metal
b) Entrapment of core and mold gases evolved under pressure
c) Reaction of carbon in the metal with oxygen or oxides.
1.3 Blowholes due to high gas content of the metal
They are also called as endogenous gas holes or blowholes. These holes are caused due to excessive gas content in the metal bath and rejection of dissolved gases during solidification. The gases involved in this defect are hydrogen and nitrogen. Both are soluble in liquid cast iron and relatively insoluble in solid iron. As casting solidifies, the insoluble gas is rejected and produces holes between growing crystals. Blowholes from carbon monoxide may increase on size by diffusion of hydrogen or less often nitrogen.
1.4 Carbon-oxygen reaction holes
The gas holes in this group may appear in a variety of forms, but the gas responsible is carbon monoxide, produced by the reaction of oxygen containing substances with the carbon present in the cast iron. Manganese sulfide in the oxide-rich liquid slag allows the reaction to take place at lower temperatures and facilitates the entrapment of gas in solidifying metal.
1.5 Blowholes from mold or core gases
These holes are caused due to excessive moisture in molds or cores, core binders which liberate large amount of gas, excessive amount of additives containing hydrocarbons and blacking, and washes which tend to liberate too much gas.
If the gas generated from molds and cores cannot freely escape, it may get trapped in the liquid metal. The bubbles formed remain in the casting during solidification. The gases producing these holes consist mainly of steam, coal gas and hydrocarbon gases from decomposition of organic core binders.
1.6 Mechanical entrapment of gas
These holes are caused due to insufficient evacuation of air and gas from mold cavity and insufficient mold or core permeability. The blowhole formation is also affected by the parameters like pouring temperature, rate of pouring, slag inclusion, moisture and clay content of mold and sand, type of binder and type of additives used etc.
1.7 Blowholes from Green Sand Molds
The principal sources of gas from green sand molds are moisture and seacoal, from which gas liberates rapidly upon heating. The escape of this gas during mold filling takes place via two routes:
1. the pores of the molding sand
2. special vent holes provided by the molder
The holes in the majority of cases are large and have smooth walls. There is optimum moisture content for sand depending upon the proportion of fines and the type of clay used. If this moisture content is exceeded, then blowholes may be produced. It is up to 4 percent for sands containing fireclay and bentonite. The permeability of the naturally bonded sand is usually low and the effect of moisture is to reduce this even further, at the same time increasing the gas producing potential of the mixture. It is more dangerous than the use of lower permeability base sand at lower moisture content. The addition of sea coal will also reduce the permeability of molding sand and increase the quantity of gas produced. Further, sand having high sea coal content requires more moisture to bring it to a workable condition. A study on behavior of gases in case of a plane wall in dry mold indicates a rapid buildup in pressure at the mold metal interface. Slow pouring leads to a low pressure after filling the mold. Consequently, gas which enters the metal before the mold is full is aided in escaping by metal movement.
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