1 4501 HEAT TREATMENT CRACK – HEATING RELATED I. APPLICABLE TO… II. IDENTIFICATION III. EXPLANATION IV. LITERATURE V. ROOT CAUSES & SECONDARY CAUSES VI. PROCESS CAUSES VII. ACTIONS ON ROOT CAUSE VIII. ACTIONS ON PROCESS CAUSE Gietech BV has to its best knowledge compiled the file and cannot be hold responsible for problems if applying the data and actions without proper control, evaluation and surveillance of the reader. Gietech BV can assure that the data are collected, evaluated and exposed with all the experience of Staf Henderieckx, its owner. The reader should realise that the author does not know the conditions in the reader’s foundry, which indicates that a particular verification and selection of solutions can be required. NO INFORMATION FROM THIS FILE CAN BE COPIED, MULTIPLIED OR SOLD WITHOUT THE WRITTEN PERMISSION OF GIETECH BV, WHICH HAS THE TOTAL COPYRIGHT OF THIS FILE. 2 I. APPLICABLE TO… STEEL IRON Carbon steel Gray iron Low- & medium-alloyed steel Ductile iron High alloyed steel Compacted iron Corrosion resistant steel Malleable iron Heat resistant steel White iron Manganese steel Gray iron alloyed Chromium steel Ductile iron Austempered ADI Other steel Ductile iron Ni-alloyed Ductile iron Silicon alloyed White iron Cr-Ni alloyed White iron Cr-alloyed Other iron ALL STEEL x ALL IRON x MOULDING Green sand hand moulding Ceramic moulding Green sand medium pressure Ceramic moulding lost wax Green sand high pressure Metal mould No bake sand Centrifugal casting Vacuum moulding Lost foam Plaster mould Full mould (foam) All MOULDING x MELTING Cupola cokes Arc furnace Cupola (cokeless) gas Main frequency furnace Cupola duplex Induction furnace medium Hz Rotary furnace Air Induction furnace high Hz Rotary furnace oxygen Channel furnace Flame fired crucible furnace Vacuum furnace All MELTING x 3 II. IDENTIFICATION II.1 TOOLS II.2 DESCRIPTION II.3 PICTURES II.1 TOOLS IDENTIFICATION TOOLS NDT – VT x Destructive testing x NDT- Microscope x Mechanical testing NDT- PT x Physical testing NDT-MT x Measuring dimensionally hand NDT-UT x Measuring dimensionally 3-D NDT-RT x Measuring dimensionally laser NDT-Other x Analysis testing: spectrometer Roughness test Analysis testing: other ANY TESTING Paperwork 4 II.2 DESCRIPTION LOCATION related to Cope / drag Chill location % presence % presence Riser location Vent location % presence % presence Pouring system (ingates) NO RELATION x % presence The defect shows a crack that does exhibit an oxide skin and is mostly open type (clear distance between crack walls), also called hot crack, hot tear and hot shortness. The cracks can be small, larger and deep up to, especially for thick sections and extreme cases, broken casting section. The crack walls are more rough compared to the cold cracks, which are flat and smooth. Some literature make a difference, telling that a hot crack is occurring after solidification and a hot tear during the solidification. In this text, there will be no difference considered. It happens when the castings are heated for the heat treatment and during the stay at high temperature. The crack will mostly appear in sharp edge connections or near other stress raising details (in between ribs, risers…) . The crack is never in the center of the material section and or parallel to the surface walls. It is perpendicular on the casting surface. Defects 42xx are similar defects, but they are initiated at low temperature and have no oxidized crack walls. Defect 43xx looks similar but differentiates by being low depth and having a rounded crack root (base). Sometimes, the casting will also be deformed on top of the cracks. Deformation belongs to the 8-group of defects. 5 II.3 PICTURES II.3.1 Principle II.3.2 Castings Heat treatment crack – heating operation II.3.3 NDT tests Heat treatment crack – RT tested Heat treatment crack – heating operation Heat treatment crack – heating operation Hot crack – RT tested Hot crack – MT tested 6 Hot crack – UT testing 7 III. EXPLANATION III.1 EXPLANATION III.2 REQUIRED INFORMATION III.1 EXPLANATION The crack is initiated during the heating and the stay on temperature. The crack is open type (clear gap between the crack walls) and has rough (compared to cold crack) crack walls. It has completely oxidized walls due to the long stay on high temperature. This defect appears due to several reasons and or a combination of them. A crack is always requiring a tensile stress, mostly present at the casting surface. For this reason, it mostly starts at the surface of the casting, very seldom inside the section unless there is a stress raising defect (porosity, inclusions, dross) or a large amount of carbides. During the heating, the residual stress (from pouring, cooling and other operations) will be increased by the thermal stress due to the temperature differences of the different casting sections. The shape of the casting, especially complex shape with thin and thick sections, as well as the sharp connections of different sections sizes will increase the hot crack presence. The amount of thermal stress depends mainly on temperature differences, equal to: σth = k x α x E x ΔT k: factor depending on material and shape of component α: linear expansion factor of the material (mm/mm.°C) E: modulus of elasticity (Young’s modulus, MPa) ΔT: temperature difference between sections, which depends on the shape, heat density and heat conductibility of the material (°C). At the same time, due to the higher temperature, some stress relieving will happen, decreasing the risk for cracking. The dimensional and volume changes, due to the temperature increase and potential structure transformations, will also increase the stress level. The following example is for carbon steel: Reaction Volume change, % Dimensional change, in./in. Spheroidite to austenite -4.64 - 2.21 (%C) 0.0155 - 0.0074 (%C) Austenite to martensite 4.64 - 0.53 (%C) 0.0155 - 0.00118 (%C) Spheroidite to martensite 1.68 (%C) 0.0056 (%C) Austenite to lower bainite (a) 4.64 - 1.43 (%C) 0.0156 - 0.0048 (%C) Spheroidite to lower bainite (a) 0.78 (%C) 0.0026 (%C) Austenite to aggregate of ferrite and cementite (b) 4.64 - 2.21 (%C) 0.0155 - 0.0074 (%C) 8 Spheroidite to aggregate of ferrite and cementite (b) 0 0 (a) Lower bainite is assumed to be a mixture of ferrite and epsilon carbide. (b) Upper bainite and pearlite are assumed to be mixtures of ferrite and If the furnace is improperly loaded and some of the castings will be hit by the burner flame, the temperature difference will increase a lot and some de-carburizing will happen quickly. Both will facilitate this type of hot crack. The risk will be increased if the surface is de-carburized, leaving a surface layer with less carbon and less strength. Mostly it will also have some fine cracks, which will be oxidized during the treatment and will growth quickly. Other surface defects (inclusions, high roughness…) will also increase the crack presence. In case of steel, the defect can be repaired by welding if the applicable standard does allow it. It is also possible that the customer does not allow it. The customer should always be informed and preferred to give a written approval. In case of irons, the casting will be rejected. 9 III.2 REQUIRED INFORMATION REQUIRED INFORMATION PRODUCTION Material Melting & pouring Chemistry (C, Si, Mn, S, P) Highest temperature in furnace Chemistry (alloying elements) Time on temperature (> Tg) Chemistry (residual elements) Number of taps Other: Temperature between taps Metallurgical treatment: type Moulding Result O-, H-, N-content Green sand: clay, burnt clay Result Al-, Ti-content Green sand: green strength Result Mg-, RE-content Green sand: strength Time treatment to pouring Green sand: water content Pouring temperature Green sand: permeability Pouring time Green sand: sand/metal ratio Thermal analysis No-bake sand: type of sand Ladles No-bake sand: sieve analysis Heat treatment No-bake sand: shape of sand Heat treatment cycle x No-bake sand: LOI Cooling No-bake sand: LOI * Condition cooling device No-bake sand: S-content No-bake sand: N-content Shot blasting / fettling No-bake sand: P-content Shake out temperature No-bake sand: Acid demand Type of shot blasting No-bake sand: sand/metal ratio Type of PS / riser removing No-bake chemicals Type of fettling Type of coating & liquidiser Coating thickness / layers Quality control Coating drying Report NDT-test x Ceramic sand: type of sand Tensile test result Ceramic sand: mould thickness Hardness test result Ceramic sand: drying type & time Charpy-V test result Ceramic sand: wax-removing Other tests Ceramic sand: pre-heating Pouring system & risers & chills Metal mould: type and thickness Pouring system design Metal mould: coating type First metal Metal mould: coating thickness Venting Mould: pre-heating Pouring box Other info Type of risers Weather conditions: humidity Type of chills Weather conditions: temperature Type of sealing 10 IV. LITERATURE Process Control Manual G Henderieckx, Gietech BV, soft copy Rejection & Rework manual G Henderieckx, Gietech BV, soft copy B 110104 Cracks in steel castings G Henderieckx, Gietech BV, soft copy
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