Analysis And Explanation Of Defects In Cast Iron Parts

by | Jun 22, 2022 | Industry Knowledge

Defect analysis of cast iron is a common item in metal inspection. The reason is that in the process of processing or using iron castings, there may be certain appearance quality, internal quality and use quality problems, and the quality of cast iron parts will have a great impact on the properties of mechanical products, once defects.

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It is likely to lead to mechanical accidents, thus affecting the overall quality and working life of products. There are many reasons for the defects of cast iron, and the forms of defects also have their own characteristics, so through defect analysis, we can quickly find out the causes of defects, so as to improve the production process, or improve the technical level.

Most of the defects of iron castings occur in the processing process. Due to the complexity of the process of iron castings, whether it is structural design, casting raw material quality, or process operation may affect its final quality performance. At present, there is no standard to clearly describe the classification and naming of casting defects in China.

Once the manufacturer and buyer misunderstand because of casting defects, it is difficult to find the relevant standard basis. Of course, the state also paid attention to the related problems, began to solve the relevant classification and description of cast iron defects, the following collated some common cast iron defects, detailed introduction to you.

I. Meat defects

These defects can be divided into three categories, including flying wings or flying edge succulent defects, bulk succulent defects and other multi-meat defects.

The following is an introduction to common defects:

  1. Fleshy defects of flying wings or flying edges
    (1) do not change the basic size of the casting: the thin flying wing (or flying edge) on the parting line or core head, the pulse protruding pulse pattern on the casting surface, the reticulated flying wing on the die casting surface, the thin flake raised needle wing parallel to the casting surface at the inner angle, the sheet metal protruding at the inner corner, and the inner angle flying wing in which the fillet is divided into two parts.
    (2) the main dimensions of the castings are changed: the thick flying wings of the castings at the parting line, the cracked thick flying wings at other positions of the castings, and the cracking of the mold shell of the investment mold precision casting, and the castings produce a large area of thin flying wings.
  2. Large pieces of succulent plant
    (1) Sand expansion: there is excess metal on the inner and outer surface of the casting, that is, the sand expansion on the outer surface or the inner surface, the sand flushing with excess metal near the inner gate or the bottom of the direct gate, and the combined compression of the metal succulent in the closing direction.
    (2) rough succulent meat on the surface of the casting: rough succulent on the upper mold surface of the casting; succulent on the lower mold surface of the casting, that is, the core (core) cracking and floating core; rough succulent (dispersed) on the lower mold surface of the casting, that is, the bottom scar; rough succulent meat in other parts of the casting, that is, sand falling off; rough succulent meat on the large area of the casting surface, that is, the inner corner sand scar; the surface succulent formed by the sand core. That is, sand core crushing.
  3. Other meat defects
    A small piece of metal succulent defect with a smooth surface: a similar spherical succulent formed on the surface of a casting, at a right angle, or at some inner wall (exudate). This kind of defect is generally exudate, phosphide bean, tin infiltrated bean, lead infiltrated bean.

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II. Hole defects

Hole defects are also roughly divided into three types of defects, namely, circular holes with smooth walls (pores, pinholes), rough pores in the inner wall, shrinkage pores and porous structures caused by a large number of small pores. The following is an introduction to common defects:

  1. Holes (pores, pinholes) visible in the eye and generally round with smooth walls
    Holes are usually in the casting wall, not extended to the casting surface, can only be distinguished by special methods, equipment or broken castings: circular holes similar to pores and pinholes, usually smooth and varying in size, single or group irregular in all parts of the casting; or limited to the metal sheet into the near area of the mold (cold iron, embedded castings, core brace, etc.); or with slag holes.
    Holes located on or close to the surface of a casting, most of which are exposed or at least connected to the surface of the casting: such holes are usually of different sizes, single or group, usually located on or near the surface, and the inner wall is shiny; or located at the inner corner of the casting, often extending to the corner of the casting; surface pinhole defects, which may be small pores (holes) on the surface of the casting, extending more or less. Such defects can usually be found after sandblasting cleaning or machining; cracked pinholes are usually cracked small narrow holes that exist on the surface or edge and can be found only after machining.
  2. Rough holes and shrinkage holes in the inner wall
    Sometimes open holes extending to the interior of castings: open shrinkage holes are usually open funnel holes with dendritic crystal structure on the inner wall; inner angular shrinkage holes, usually holes with sharp edges, located at the fillet or gate position of thick castings; core shrinkage holes, usually holes extending from the sand core.
    Complete internal hole in casting: internal shrinkage, usually irregular, and pore wall usually dendritic structure; centerline shrinkage, usually along the central axis of the hole or porous area
  3. Porous structure caused by a large number of small pores
    The holes are almost invisible to the naked eye: macro shrinkage, micro shrinkage, shrinkage loosening, casting leakage, usually branch shrinkage pores scattered on the casting section.

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III. Discontinuous defects

Discontinuous defects are generally referred to as discontinuity defects caused by machining effects. Discontinuity defects usually occur at intersections, fractures. According to the shape and appearance of the fracture, the fracture is not likely to be caused by internal stress.

  1. Ordinary crack: mechanical cold crack, belongs to the general appearance of fracture, sometimes adjacent area with indentation;
  2. The existence of oxidation of the crack: mechanical hot crack, usually fracture surface or edge of the complete oxidation;
  3. Discontinuous defects (cracks and tears) caused by internal stress and internal contraction: stress cold cracking, which is generally a square edge fracture formed by the influence of tensile stress in the cooling process, and the fracture surface is not oxidized; Stress-thermal cracking is a kind of fracture without regular shape in the stress area, and the surface of oxidation fracture is dendritic. Quenching cracks, generally in the rapid cooling or heat treatment process after the complete solidification cracking;
  4. Discontinuous defects caused by unfused (cold spacer): the edge is usually round, indicating that the different metal flows in the mold filling process are not fused properly, and the edges are usually round;
  5. In the process of filling the cavity last part of the defect is not fully fused: cold separation, usually refers to a vertical surface, the casting section is completely or partially separated;
  6. Casting between the two parts of the poor fusion defect: horizontal cold interval, generally refers to the horizontal plane of the casting separated;
  7. Defects of poor fusion around core support, internal chill and embedded casting: poor fusion (cold separation of embedded casting parts), generally expressed as local interruption near metal embedded casting;
  8. Discontinuous defects caused by metallurgical defects
    Separation along grain boundaries:
    Grain boundary embrittlement (conchoidal or crystal sugar fracture) is characterized by primary crystallization separation along grain boundaries.
    Intergranular corrosion, which shows net-like cracks throughout the section, is usually a defect in zinc alloy die casting.

IV. Surface defects

Common surface defect

  1. Casting surface fold defect: wrinkle skin, refers to the casting large area fold; Like skin fold skin, refers to the surface of a network of uneven folds; Creases (snake-like creases), refers to the continuous wavy folds, the edge of the folds in the same plane, the casting surface is smooth; Flow mark, refers to the casting surface has the trace of metal flow (low alloy);
  2. Surface roughness: The depth is similar to the surface roughness of sand grain size; Surface roughness, refers to the depth of the surface roughness greater than the size of the sand, such defects are generally high-pressure molding defects;
  3. Casting surface concave groove: groove, refers to the length of different and mostly branched concave groove, its bottom and edge smooth; Rat’s tail refers to a groove up to 5 mm in depth, with one edge forming a fold that more or less covers the groove; Fishtail flow mark, refers to the irregular distribution on the surface of the casting of different sizes of indentations, usually along the metal flow path distribution, such defects often exist in cast steel; Orange peel pockmarked, refers to the entire casting surface filled with pits or pockmarks; Sticking type refers to the grooves and roughness near the inner angle of the die-casting part;
  4. Casting surface depression: shrinkage, refers to the hot spot near the surface of the casting depression; Lacuna refers to the droplet or shallow punctate holes on the surface of a casting, usually gray-green, which occur in investment chromium carbon steel.
    Severe surface defects
  5. Casting surface deep depression: collapse, squeeze box, refers to the deep depression, usually accounts for most of the casting under the mold;
    Sand sticking, more or less vitrification: chemical sticking, refers to the sand layer firmly adhered to the casting surface; Hot bonded sand, refers to the casting surface adhesion of partially melted sand; Mechanical sand, refers to the hottest parts of the casting (inner corner and core), sand and iron firmly bonded aggregates; Shell spalling, refers to the mold material fragments embedded in the casting surface, often occurs in investment casting.
  6. Rough sheet metal bulges, usually parallel to the surface of the casting: sand scarring refers to the rough sheet metallic bulges that are parallel with the casting surface, which can be removed with a chisel;
    Peeling scar, similar to sand scar, but need to be machined or polished to remove; Coating scarring, refers to the sand or sand core coating exfoliation, so that the casting surface of the flat metal convex;
  7. Oxide adhesion after decarbonization heat treatment (annealing, tempering, forging annealing): oxide scale refers to oxide adhesion following annealing; Filler melting, refers to the ore adhesion after malleable annealing, often occurs in white heart malleables; Annealed scale refers to the formation of oxide scale after being malleable annealed.

V. Incomplete defects

  1. Part of the casting is missing no break)
    Casting relative pattern appearance deviation: casting insufficient, refers to in addition to some edge, corner location slightly circular, casting basic integrity; Poor coating, improper modification, due to improper modification or coating application, resulting in casting edge or contour deformation;
    The relative pattern of casting has serious deviation: serious underpouring, which means that the casting is incomplete due to early solidification; Incomplete casting due to insufficient liquid metal; Fire running, mold leakage (leakage box), refers to the casting is incomplete due to the loss of liquid metal from the mold after pouring; Overshot blasting, which means that the material is seriously missing due to excessive shot blasting; Heat treatment melting or deformation, refers to the heat treatment process casting partial melting or severe deformation;
  2. Part of the casting is missing with breaking:
    Casting fracture, refers to the casting broken, large missing; The fracture surface is not oxidized; Casting small pieces of broken, generally runner, riser with meat (ingate, risers or outlet fracture); Casting fracture, with oxidation fracture, usually high-temperature fracture, mainly fracture surface exposed at high temperature oxidation.

VI. Size or shape error

  1. Casting all dimensions are not correct: shrinkage rate selected wrong, as in the same proportion, all dimensions of casting is not correct;
  2. Casting some size is not correct: shrinkage is blocked, the performance of the long size is too large; Irregular shrinkage, manifested as some dimensions are not correct; Pattern loosening is too large, showing that the size of the starting die is too big (in 1, 2 or 3 directions); Sand baking expansion, performance is large perpendicular to the parting surface direction of the size is too large; Sand mold is not Chong tight, cavity expansion, performance for the casting surface irregular location of the thickness increased; Appearance Chong deformation, performance for casting general wall thickness thinning, especially in the horizontal plane;
  3. Pattern is not correct: pattern error, refers to some parts of casting parts or many parts do not meet the drawing shape, pattern does not meet; Pattern assembly error, refers to the individual parts of the casting shape and drawings are different, pattern is correct;
  4. Wrong type: wrong type (wrong box) refers to the casting in the parting surface of the two parts of each other staggered; Wrong core, refers to the casting cavity shape along the core parting surface changes; Pounded, refers to the casting vertical surface irregular convex, usually only on the side adjacent to the parting line.
  5. Deformation: pattern deformation, refers to the casting, sand mold, pattern and drawing proportional deformation; Sand deformation, refers to the casting, sand and drawings proportional deformation, pattern and drawings match; Casting deformation, refers to the relative deformation of casting drawings, pattern, mold and drawings in line; Casting warpage refers to casting deformation relative to the drawing after storage, annealing and machining.

VII. Inclusions or tissue abnormalities

  1. Inclusions: metallic inclusions, mostly inclusions, appearance, chemical composition or structure inspection shows that it is caused by foreign elements entering the alloy; Cold beans are metallic inclusions whose chemical composition is the same as that of the base metal, and are usually spherical and have an oxide layer on the surface. The phosphide is usually a spherical metallic inclusion in the pores or in other holes or surface depressions, and the composition is close to the alloy casting but closer to the eutectic composition;
  2. Slag, dross and flux non-metallic inclusions: slag inclusion, usually slag, slag or flux; Being non-metallic inclusions whose appearance or composition indicates that they were produced by slag, metal treatment products or flux; Gas-bearing slag pores, which are non-metallic inclusions, usually containing gas, with pores;
  3. Core material non-metallic inclusions: trachoma, a sand inclusion, generally close to the casting surface; Coating inclusions belong to sand coating inclusions, generally near the casting surface;
  4. Non-metallic inclusions in oxides and reaction products: black slag, clear irregular black spots (magnesium silicate) on the fracture surface of nodular cast iron; Oxide inclusions, or oxide scale inclusions, often produce local cracks; Bright carbon film refers to the shiny, wrinkled graphite film on the inner wall of the casting; The hard spot, refers to the hard inclusion in the metal mold casting and gravity die casting aluminum alloys.
  5. Macroscopic tissue abnormalities
    (1)Abnormal organization of gray iron, nodular cast iron, worm cast iron: white mouth, refers to part or all of the organization is white, especially the thin wall, prominent outer corner and edge, gradually transition to normal organization; The transition zone without hemp was white, which was similar to white, but there was no transition to normal structure. Reverse white, refers to the casting part of the final solidification of a clear white area, the surface organization is normal.
    (2) Malleable iron structure abnormal: Hemp mouth, refers to the black spot in the as-cast structure, heat treatment fracture into coarse crystal, gray black; White edge refers to black heart malleable cast iron. After annealing, there is an obvious bright layer over 0.5mm on the fracture surface, and its interior is black; Local hard spot refers to the thin hard surface layer containing hardened tissue;
    (3) special-shaped graphite structure: graphite thick, refers to the uniform distribution of thick flake graphite; Coarse floccule graphite refers to the existence of local accumulation of coarse flake graphite in the organization, and the presence of graphite deposition in the shrinkage cavity; Graphite floatation refers to the presence of graphite pellets on the surface of the casting. The dendritic crystal structure refers to the nodular cast iron fracture surface with random direction.

These are the common defects of cast iron, according to different circumstances. We can divide the defects into seven categories: meaty defects, holes defects, discontinuous defects, surface defects, incomplete defects, size or shape errors, inclusions or abnormal tissue. Of course, these deficiencies, we also subdivided into sub-categories and explained.

When we do defect analysis, not only through the appearance evaluation.But also through metallographic analysis, non-destructive testing and other ways, the internal organization of scientific determination.

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