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Introduction

Over the past 10 years steel makers worldwide have developed new structural grade steels with higher yield and tensile strengths. These developments have enabled manufacturers to design their steel products using lighter section steels which in turn reduces the production, transport and erection costs of the finished product. Prior to these developments, the steel fabrications which were most commonly galvanized were manufactured from Grade 250 mPa hot rolled structural steels.

Since the early 1970’s, the results from research and testing centres around the world have shown that the hot dip galvanizing process does not affect the tensile and proof (yield) strengths of the Grade 250 mPa structural steels. But does the hot dip galvanizing process affect the yield and tensile strengths of the newer high-tensile grades of structural steels?

Industrial galvanizers has been asked these questions on a number of occasions following claims made by others that galvanizing of these steels affected their tensile strength and performance. To ensure that factual information was available, Industrial Galvanizers undertook a testing program of galvanizing these steels to establish the effect of hot dip galvanizing on their structural performance.

Objective: The aim of this test program was to establish if the hot dip galvanizing process of dipping steel in molten zinc adversely affects the strength properties of a typical high tensile steel using standard hot dip galvanizing practices. These practices include duplicating the immersion time of the steel in the molten zinc (this does not exceed 15 minutes under normal conditions) at a temperature of 455 degrees Celcius.

Test 1 Product: HA70T-P Hot rolled, with black finish manufactured by BHP Steel.

HA70T-P hot rolled steel has a guaranteed minimum yield strength of 450 mPa and a minimum hardness of 70 HRB. The typical yield strength is between 520 to 610 mPa. The typical tensile strength is between 530 to 620 mPa. This steel is normally used in shelving, automotive parts and more recently for purlins.

Procedure: The test procedure involved cutting eleven pieces from a of 3.0 mm thick black HA70T-P steel coil. Six of the pieces were hot dip galvanized in accordance with AS/NZS 4680-1999. The remaining pieces were left in black (as rolled) finish. All sections were then delivered to the BHP Port Kembla Technical Services for testing.

Test 2 Product: GALVASPAN G450 Zinc coated, structural grade manufactured by BHP Steel.

GALVSPAN G450 has a guaranteed minimum yield strength of 450 mPa and is an in-line hot dip zinc coated structural grade steel. The typical yield strength is between 470 to 550 mPa. The typical tensile strength is between 510 to 600 mPa. This steel is normally roll formed into products such as purlins, girts and light structural profiles.

Procedure: The test procedure involved cutting six pieces from a single length of a roll-formed Z25024 purlin, which had been roll formed by BHP Building Products. The steel thickness is 2.4 mm. Three of the pieces were acid pickled (to completely remove the Z350 mill applied zinc coating) and hot dip galvanized in accordance with AS/NZS 4680-1999. The remaining pieces were left in the mill applied Z350 Zinc coating (as rolled) finish. All sections were then delivered to the BHP Port Kembla Technical Services for testing.

Results

Tables 1 and 2 show the results of the testing . If the results of the yield strengths of the black sections and the results for the galvanized sections of the HA70T-P are averaged, the difference is 0.4%. As this variation is less than 1% it is considered to be within the accuracy tolerance of the testing procedure.

If the results of the yield strengths of the black sections and the results for the galvanized sections of the GALVASPAN G450 are averaged, the difference is 0.6%. As this variation is less than 1% it is considered to be within the accuracy tolerance of the testing procedure.

Both these tests have verified that hot dip galvanizing of either un-coated high strength steel or the hot dip galvanizing of pre-galvanized high strength steel after acid stripping of the original coating has no effect on the structural strength of the steels involved.

Approximately 100,000 tonnes of steel in various forms, such as this large navigation tower is galvanized annually by Industrial Galvanizers without any problems relating to the hot dip galvanizing process affecting the steel's strength