Hardenable Steels Alloy Carbon > 1% 0.15-0.8% The hardenable steels are defined as those steels having total alloy content greater than 1% and carbon levels between approximately 0.15 to 0.8%. Some hardenable steels actually go to carbon contents above 0.8% and get into a hypereutectoid grade but we will only marginally consider these steels.

Resistance Welding Learning Activities View Slides; Lesson Objectives Read Notes, Listen to lecture Do on-line workbook Lesson Objectives When you finish this lesson you will understand: Keywords

The American Iron and Steel Institute and the Society of Automotive Engineers have a code system to designate these alloy steels based upon their major alloying element(s). This system is presented here.

There are a number of reasons for adding these alloying elements to steel as listed here. In some cases they are added to purify the steel during melting or casting, in some cases they increase hardness or strength or increase the hardenability. In some cases they increase toughness or creep resistance. Many times these additions have some adverse effect on the weldability of these alloys. Where this is the case, these adverse effects are also listed here.

Metallurgical Considerations 1.2% C 0.8% C 0.35% C Finer Grain Cobalt Mn, Ni, Mo, C, Cr There are some universal metallurgical considerations which are a function of increasing alloy content as illustrated here. With the addition of most alloys to steels (with the exception of Co) the nose of the CCT diagram is moved to the right, making the formation of martensite easier. Only cobalt and the presence of finer austenite grain sizes move the nose to the left. A second fact is that once martensite is formed, increasing the carbon content increases the hardness and strength of that martensite. Finally when tempering fully hard martensite, the strength drops off as seen in the bottom schematic. Some steels have a slight secondary hardening at first temper due to some precipitation hardening, but this rapid diminishes with higher tempering temperature s or longer times. Metallurgical Considerations 1.2% C 0.8% C 0.35% C

Factors to be Considered When Welding Hardenable Steels Surface Cleaning Weld Force Weld Current Weld Time Upslope Downslope Indentation Preheat Post Weld Temper

Surface Cleaning Scale usually more tenacious (especially Cr, Cu) Surface Cleaning must be more complete Pickle (preferred) Grind / Polish Shot Blast

Weld Force Alloy Grades have: Higher Strength Than Plain Carbon Steels High Hot Hardness Than Plain Carbon Steels Therefore: An increase in weld force (up to 10% greater) is needed over plain carbon steel at the same gage

Weld Current Base Metal Resistance is Higher Than Plain Carbon Steel Because of Higher Alloy Content Therefore: Lower Currents are needed (approximately 90% of that required for Plain Carbon steel)

Weld Time For Lower Alloy Grades: A longer weld time by as much as 2 to 3 times helps. Short Time Higher Current Rapid Cooling Long Time Lower Current Slower Cooling The combination of weld parameters which give acceptable size weld nuggets can be adjusted at a short time-high current set of parameters or alternatively at a ling time-lower current set. With the longer time set, there is more heat affected zone heat soaking which results in slower cooling rates as illustrated. If the nose of the CCT curve is not too far moved to the right, some benefit of the formation of softer ferrite pearlite structures rather than the hard martensitic structure can occur. With higher alloy grades this is not effective. For Higher Alloy Grades: Nose is too far back and longer weld time is not effective

Indentation Long Time Lower Current Slower Cooling The longer time set of parameters can cause some problems with electrode indentation and electrode life, however.

Upslope Helps Set Electrodes Helps Bring Parts Together In some cases, the use of upslope in the current can help as it helps set electrodes into the very strong alloy sheets, thus bringing the parts together. Helps Set Electrodes Helps Bring Parts Together Some slight enlargement of HAZ but no great effect on Cooling Rate

Downslope With Downslope Without Downslope Downslope is of benefit as this controls the cooling rate from the molten nugget. With downslope, the cooling rate can be forced to fall in the ferrite pearlite range.

Effect of Quench Time and Temper Heat on Spot Weld Weld Time Temper Time Temper Heat Quench Time See the demo on the web page for explanation Effect of Quench Time and Temper Heat on Spot Weld

Short Quench Time For short quench time, there is a possibility that the nugget never reaches the martensite start temperature and thus no martensite is formed. If this is the case, then tempering cycles will not be effective at all.

Minimum Quench Time Thicker Material 1045 1035 1020 1010 An increase in the carbon content in the alloy not only increases the as quenched hardness of the martensite, but it also lowers the martensite start and martensite finish temperatures. With these temperatures lowered, a longer quench time is required to be sure that the full traverse of this transformation range is reached. In some cases, the martensite finish temperature may be below room temperature and a refrigeration might be required to complete transformation. Increase Carbon Moves Range Down

Minimum Temper Heat NiCrMo (86xx) CrMo(4xxx) More Resistant To Tempering Plain Carbon Some alloy steels are more resistant to tempering, and thus a higher temper heat may be required.