Quality Assurance

Thickness Testing

Several methods are used to determine the thickness of the zinc coating on a hot dip galvanized article. The size, shape and number of pieces to be tested, will dictate the method to be used. Specified test methods are either destructive or non-destructive.( These are detailed in ISO 1461 and in EN 10240 ). The most practical test is the non-destructive method utilizing the electromagnetic principle for determining coating thickness.

Threaded articles must fit their mating parts and, in the case of assemblies that contain both externally and internally threaded articles, shall be possible to screw mating parts together by hand.

For small items, particularly those with complex geometries, ISO 1460 provides for gravimetric measurements aimed at determining mass of coating per unit area as opposed to thickness.(This is a destructive test method) .

  Appearance

The ability of a hot dip galvanizing coating to meet its primary objective which is to provide corrosion protection, should be the chief criteria when evaluating coating acceptability.

The specified requirements for a hot dip galvanized coating are that it be :

• Continuous,
• Relatively smooth,
• Free from gross imperfections,
• Free from sharp points (that can cause injury)
• Free from uncoated areas

The above requirements are of particular importance when a subsequent organic paint coating is to be applied onto a galvanized surface. Smoothness and lack of roughness achieved on mechanically wiped products, such as continuously galvanized sheeting or wire, are not to be used as the criteria for accessing surface finish on general hot dip galvanized products. Roughness and smoothness are relative terms. The end use of the product must be the determining factor in setting standards.

Test Mechanism

There are a number of simple magnetic gauges that can be used to give a convenient and reliable measurement of the zinc coating thickness, provided the instruments are properly calibrated.
The three most common types of metal coating thickness gauges are:

• Magnetic balance gauges, sometimes called "banana gauges," measure variation in the force of attraction between two ferromagnetic bodies as a function of the distance between them. This type of gauge has the advantage of being able to measure the coating thickness in a horizontal or vertical position.
• Pull-off magnetic gauges are also based on magnetic attraction to the underlying steel. These devices are shaped somewhat like a pen and are very convenient to make quick, rough estimates to determine whether the coating thickness meets specification.
• Electronic gauges are the easiest and most accurate coating thickness measurement gauges available. They have the ability to connect to an assortment of probes, providing the ability to measure on any orientation.

One of the major advantages to specifying hot-dip galvanized steel is the ease of identifying coating defects immediately after galvanizing. Any areas that may remain uncoated are easily identifiable. If large areas (see ASTM A 123) of the steel remain uncoated due to residues left on the steel from fabrication, then the steel must be stripped free of zinc and processed again. If small areas are left un-galvanized, they can be reconditioned using one of the three accepted methods of touch-up and repair, (see ASTM A 780) .

  Defects on the Surface of Galvanized Sheets :

Defects on galvanized steel sheets can be due to steel defects, galvanizing process errors or a packing and carrying errors . Listed below shown you some of most common defects on galvanized sheets :

 

Defects	Descriptions
Rough Surface	. Steel was already rugged before galvanizing . ( Note : hot dip galvanizing makes roughness more visible ) . High percentage of phosphorous (P) or silicon (Si) in steel composition . . High coating thickness .
Dross Protrusions	Surface appearance slightly rough due to zinc dross . This is a process error cause of big structures agit   Page  1     2    ates dross from the bottom of zinc bath . ( It's not grounds for rejection )
White Rust	These large white-gray stains are due to a wet storage of manufactures. They are composed by zinc-oxide and hydroxide and they can be "erased" by brush.
Low Coating	material is absolutely prime however only the zinc coating is lower as compared to the original specified order. ( or Pin type uncoated spots )
Scale Rolled	Minor grey colors spot at interval .
Adherence	Normally zinc adherence is tested on 0T however in this case it failed at 0T but passed at 1t position .
Under Weight	Lower coil weight produced against specified ordered coil weight .
Roll Mark	Minor crease line along the length of the coil at interval.
Black Spots ( PBLS )	In this case there are some black spot arises in the sheet due to some process problems. However this black spot will be only in some portion of the coil and not all over the coil length.
Gauge Variation	The thickness variation exceeds the limits specified in the order specifications .
Dross Patches	Minor zinc dross points on surface .
Water Mark
Fold Mark	Minor crease line along the width of the coil .
Edge Wavy	Edge waviness 5 - 6 mm amplitude and 3 - 4 number of waives .
Zinc Peaks	Zinc peaks with risk of injury or flaws
Rusty Surface
Paneling Mark	Stretcher strain mark. This material is not suitable for making pipes. If you fold this material in circular shape there could be folding mark.
Ridge	Slightly elevated surface. This is related to the flatness of the material, when the flatness exceeds the specified limit of the specification.
Fili-form corrosion
Dent Marks	Minor projected dent mark on coil at interval.
scratches
doublings
Winding Fold	Minor crease line along with the width of the coil.
Nozzle Line	Minor line along the length of the coils which is only visible.
Puct	This is pin type uncoated spots on the surface.
Wrinkle	Minor crease line along the length of the coil in few portion.
R esidual Scale
M arks
Edge Bend	Edge of the coil slightly turn up like bend .
Edge Damage	Minor damaged mark on edges of the coil .
Gauge Variation	In this case the thickness tolerance exceeds the permissible thickness tolerance specified in the original order.
Cross Bow
Buckles
Waves
Camber
Skin Breaks
Splinter

 Bending Test
Bending test is performed to determine the ductility of steel sheet. Test specimen for cold-rolled steel sheet is designated in KS B 0801 No.3. In the test, the specimen is bent to a specified angle on a mandrel or a specified radius until fracture. The ductility of the sheet is judged by the cracks on the outside of the bent specimen. In case of cold-rolled sheet the specimen is bent 180 degrees.
 Hardness Test
Harness of steel sheet is closely related to other properties like strength, wear resistance, and workability.
Hardness test is a good indicator of various properties of steel sheet. For measuring the hardness of cold-rolled steel sheet, Rockwell Hardness test is used.

Rockwell Hardness Test
Material resistance to indentation is a qualitative indication of its strength. Steel ball or diamond is often
used as an indenter. With this system, hardness value is determined by the difference in penetration depth
from an application of initial minor load onto the indenter followed by a major load. B-scale uses1/16(1.588mm) steel ball indenter and major load is 100kg. The B-scale most accurately measures specimens with the thickness of 0.762mm(0.030in.) or over. F-scale uses 1/16(1.588mm) steel ball indenter and major load is 60kg. It is preferable to use the F-scale for specimen with the thickness under 0.762mm.

Drawing Test
The draw ability of cold-rolled steel sheet can be measured with various testing methods. Drawing involves many intricate fabrication processes, thus it is nearly impossible to determine the drawability of a steel sheet by just applying one testing method. The following methods are most commonly used.
Erichsen Test
This test measures the draw ability of steel sheets. Steel sheets of 0.1mm~2.0mm thickness are used for testing. A test piece is punched down with a 10mm-radius steel ball plunger until the test piece cracks. Value h, measured when fracture occurs, is the Erichsen value. Usually, a larger Erichsen value means
better draw ability and workability.

Conical Cup Test
This method has gained popularity in recent years. The specimen is pressed with a flat or round puncher
as shown in the diagram. The test value is obtained by measuring the diameter of the cup which the specimen has been drawn into. Because the value closely corresponds to the actual performance of steel sheet fabrication, this test method is widely used by automobile makers.

DBTT Test: Ductile to Brittle Transition test
-Test Flow: Cup Forming (Blanking, Punching) Temperature Change Drop Weight Test
Inspect for brittle fracture (Transition temperature is the lowest temperature without fracture.)
DBTT Test Conditions (No-trimming after Forming Cup)
Classification Detail Item Conditions

Classification	Detail		Classification	Detail
Forming terms	Blank Dia.(mm)	96	Drop Weight Test	Load (kgf)	4.44
	Punch Dia.(mm)	50		Drop Heigh(m)	0.9
	Punch type	Flat Cup		Weight type	Cylinder
	Drawing Ratio	1.92		Test Piece position	Laid on to the side

* Drawing ratio range (1.7~2.16) : 85mm(1.7)~108mm(2.16))